Q:

"Failed to fetch" 404 Not Found, why?

hdaf@neoLaptop:~$ sudo apt-get update
[sudo] password for hdaf: 
Ign http*//extras.ubuntu.com saucy InRelease
Ign http*//ppa.launchpad.net saucy InRelease                        
Ign http*//archive.canonical.com saucy InRelease                    
Ign http*//archive.ubuntu.com saucy InRelease                        
Ign http*//extras.ubuntu.com saucy InRelease                         
Ign http*//ppa.launchpad.net saucy InRelease                         
Ign http*//archive.canonical.com saucy InRelease                     
Ign http*//archive.ubuntu.com saucy-updates InRelease                
Ign http*//ppa.launchpad.net saucy InRelease                         
Get:1 http*//extras.ubuntu.com saucy Release.gpg [72 B]              
Ign http*//archive.canonical.com saucy InRelease                               
Ign http*//archive.ubuntu.com saucy-backports InRelease              
Ign http*//ppa.launchpad.net saucy Release.gpg                       
Ign http*//extras.ubuntu.com saucy Release.gpg                       
Get:2 http*//archive.canonical.com saucy Release.gpg [933 B]         
Ign http*//archive.ubuntu.com saucy-security InRelease                         
Get:3 http*//ppa.launchpad.net saucy Release.gpg [316 B]                       
Hit http*//extras.ubuntu.com saucy Release                                     
Get:4 http*//archive.ubuntu.com saucy Release.gpg [933 B]                      
Get:5 http*//archive.canonical.com saucy Release.gpg [933 B]                   
Get:6 http*//ppa.launchpad.net saucy Release.gpg [316 B]                       
Ign http*//extras.ubuntu.com saucy Release                                     
Ign http*//ppa.launchpad.net saucy Release                                     
Get:7 http*//archive.ubuntu.com saucy-updates Release.gpg [933 B]    
Ign http*//archive.canonical.com saucy Release.gpg                             
Hit http*//extras.ubuntu.com saucy/main Sources                                
Hit http*//ppa.launchpad.net saucy Release                                     
Get:8 http*//archive.ubuntu.com saucy-backports Release.gpg [933 B]            
Hit http*//extras.ubuntu.com saucy/main amd64 Packages                         
Get:9 http*//archive.canonical.com saucy Release [5916 B]                      
Hit http*//ppa.launchpad.net saucy Release                                     
Hit http*//extras.ubuntu.com saucy/main i386 Packages                          
Get:10 http*//archive.ubuntu.com saucy-security Release.gpg [933 B]            
Hit http*//archive.canonical.com saucy Release                                 
Hit http*//archive.ubuntu.com saucy Release                                    
Ign http*//archive.canonical.com saucy Release                                 
Get:11 http*//archive.ubuntu.com saucy-updates Release [49,6 kB]               
Get:12 http*//archive.canonical.com saucy/partner Sources [1290 B]             
Get:13 http*//archive.canonical.com saucy/partner amd64 Packages [693 B]       
Get:14 http*//archive.canonical.com saucy/partner i386 Packages [1883 B]       
Hit http*//archive.ubuntu.com saucy-backports Release                          
Get:15 http*//archive.ubuntu.com saucy-security Release [49,6 kB]              
Get:16 http*//archive.canonical.com saucy/partner Sources [1290 B]             
Get:17 http*//ppa.launchpad.net saucy/main Sources [727 B]                     
Hit http*//archive.canonical.com saucy/partner amd64 Packages                  
Hit http*//ppa.launchpad.net saucy/main amd64 Packages                         
Hit http*//archive.ubuntu.com saucy/restricted amd64 Packages                  
Hit http*//ppa.launchpad.net saucy/main i386 Packages                          
Hit http*//archive.canonical.com saucy/partner i386 Packages                   
Hit http*//archive.ubuntu.com saucy/main amd64 Packages                        
Hit http*//archive.ubuntu.com saucy/universe amd64 Packages                    
Get:18 http*//ppa.launchpad.net saucy/main Sources [3336 B]           
Hit http*//archive.ubuntu.com saucy/multiverse amd64 Packages                  
Hit http*//archive.ubuntu.com saucy/restricted i386 Packages                   
Hit http*//ppa.launchpad.net saucy/main amd64 Packages                         
Hit http*//archive.ubuntu.com saucy/main i386 Packages                         
Hit http*//ppa.launchpad.net saucy/main i386 Packages                          
Hit http*//archive.ubuntu.com saucy/universe i386 Packages                     
Hit http*//archive.ubuntu.com saucy/multiverse i386 Packages                   
Hit http*//archive.ubuntu.com saucy/main Translation-en                        
Hit http*//archive.ubuntu.com saucy/multiverse Translation-en                  
Hit http*//archive.ubuntu.com saucy/restricted Translation-en                  
Hit http*//archive.ubuntu.com saucy/universe Translation-en                    
Get:19 http*//archive.ubuntu.com saucy-updates/restricted amd64 Packages [14 B]
Get:20 http*//archive.ubuntu.com saucy-updates/main amd64 Packages [49,4 kB]   
Get:21 http*//archive.ubuntu.com saucy-updates/universe amd64 Packages [18,1 kB]                                                     
Get:22 http*//archive.ubuntu.com saucy-updates/multiverse amd64 Packages [14 B]                                                      
Get:23 http*//archive.ubuntu.com saucy-updates/restricted i386 Packages [14 B]                                                       
Get:24 http*//archive.ubuntu.com saucy-updates/main i386 Packages [49,1 kB]                                                          
Get:25 http*//archive.ubuntu.com saucy-updates/universe i386 Packages [18,1 kB]                                                      
Get:26 http*//archive.ubuntu.com saucy-updates/multiverse i386 Packages [14 B]                                                       
Ign http*//extras.ubuntu.com saucy/main Translation-en_US                                                                            
Get:27 http*//archive.ubuntu.com saucy-updates/main Translation-en [20,5 kB]                                                         
Ign http*//extras.ubuntu.com saucy/main Translation-en                                                                               
Err http*//extras.ubuntu.com saucy/main Sources                                                                                      
  404  Not Found
Err http*//extras.ubuntu.com saucy/source Sources                                                                                    
  404  Not Found
Hit http*//archive.ubuntu.com saucy-updates/multiverse Translation-en                                                                
Err http*//extras.ubuntu.com saucy/main amd64 Packages                                                                               
  404  Not Found
Err http*//ppa.launchpad.net saucy/main amd64 Packages                                                                               
  404  Not Found
Err http*//extras.ubuntu.com saucy/source amd64 Packages                                                                             
  404  Not Found
Hit http*//archive.ubuntu.com saucy-updates/restricted Translation-en                                                                
Err http*//ppa.launchpad.net saucy/source amd64 Packages                                                                             
  404  Not Found
Err http*//extras.ubuntu.com saucy/main i386 Packages                                                                                
  404  Not Found
Err http*//ppa.launchpad.net saucy/main i386 Packages                                                                                
  404  Not Found
Err http*//extras.ubuntu.com saucy/source i386 Packages                                                                              
  404  Not Found
Err http*//ppa.launchpad.net saucy/source i386 Packages                                                                              
  404  Not Found
Get:28 http*//archive.ubuntu.com saucy-updates/universe Translation-en [10,7 kB]                                                     
Ign http*//extras.ubuntu.com saucy/main Translation-en_US                                                                            
Ign http*//ppa.launchpad.net saucy/main Translation-en_US                                                                            
Ign http*//extras.ubuntu.com saucy/main Translation-en                                                                               
Ign http*//ppa.launchpad.net saucy/main Translation-en                                                                               
Hit http*//archive.ubuntu.com saucy-backports/restricted amd64 Packages                                                              
Ign http*//extras.ubuntu.com saucy/source Translation-en_US                                                                          
Ign http*//ppa.launchpad.net saucy/source Translation-en_US                                                                          
Hit http*//archive.ubuntu.com saucy-backports/universe amd64 Packages                                                                
Ign http*//extras.ubuntu.com saucy/source Translation-en                                                                             
Hit http*//archive.ubuntu.com saucy-backports/multiverse amd64 Packages                                                              
Ign http*//ppa.launchpad.net saucy/source Translation-en                                                                             
Hit http*//archive.ubuntu.com saucy-backports/main amd64 Packages                                                                    
Ign http*//ppa.launchpad.net saucy/main Translation-en_US                                                                            
Hit http*//archive.ubuntu.com saucy-backports/restricted i386 Packages                                                               
Ign http*//ppa.launchpad.net saucy/main Translation-en                                                                               
Ign http*//ppa.launchpad.net saucy/main Translation-en_US                                                                            
Hit http*//archive.ubuntu.com saucy-backports/universe i386 Packages                                                                 
Ign http*//ppa.launchpad.net saucy/main Translation-en                                                                               
Hit http*//archive.ubuntu.com saucy-backports/multiverse i386 Packages                                                               
Hit http*//archive.ubuntu.com saucy-backports/main i386 Packages                                                                     
Hit http*//archive.ubuntu.com saucy-backports/main Translation-en                                                                    
Hit http*//archive.ubuntu.com saucy-backports/multiverse Translation-en                                                              
Hit http*//archive.ubuntu.com saucy-backports/restricted Translation-en                                                              
Hit http*//archive.ubuntu.com saucy-backports/universe Translation-en                                                                
Get:29 http*//archive.ubuntu.com saucy-security/restricted amd64 Packages [14 B]                                                     
Get:30 http*//archive.ubuntu.com saucy-security/main amd64 Packages [23,6 kB]                                                        
Ign http*//archive.canonical.com saucy/partner Translation-en_US                                                                     
Ign http*//archive.canonical.com saucy/partner Translation-en                                                                        
Get:31 http*//archive.ubuntu.com saucy-security/universe amd64 Packages [4010 B]                                                     
Ign http*//archive.canonical.com saucy/partner Translation-en_US                                                                     
Get:32 http*//archive.ubuntu.com saucy-security/multiverse amd64 Packages [14 B]                                                     
Ign http*//archive.canonical.com saucy/partner Translation-en                                                                        
Get:33 http*//archive.ubuntu.com saucy-security/restricted i386 Packages [14 B]                                                      
Err http*//archive.canonical.com saucy/saucy Sources                                                                                 
  404  Not Found
Get:34 http*//archive.ubuntu.com saucy-security/main i386 Packages [23,6 kB]                                                         
Err http*//archive.canonical.com saucy/partner Sources                                                                               
  404  Not Found
Err http*//archive.canonical.com saucy/source Sources                                                                                
  404  Not Found
Get:35 http*//archive.ubuntu.com saucy-security/universe i386 Packages [4033 B]                                                      
Err http*//archive.canonical.com saucy/partner amd64 Packages                                                                        
  404  Not Found
Get:36 http*//archive.ubuntu.com saucy-security/multiverse i386 Packages [14 B]                                                      
Err http*//archive.canonical.com saucy/source amd64 Packages                                                                         
  404  Not Found
Err http*//archive.canonical.com saucy/partner i386 Packages                                                                         
  404  Not Found
Hit http*//archive.ubuntu.com saucy-security/main Translation-en                                                                     
Err http*//archive.canonical.com saucy/source i386 Packages                                                                          
  404  Not Found
Ign http*//archive.canonical.com saucy/partner Translation-en_US                                                                     
Hit http*//archive.ubuntu.com saucy-security/multiverse Translation-en                                                               
Ign http*//archive.canonical.com saucy/partner Translation-en                                                                        
Hit http*//archive.ubuntu.com saucy-security/restricted Translation-en                                                               
Ign http*//archive.canonical.com saucy/source Translation-en_US                                                                      
Ign http*//archive.canonical.com saucy/source Translation-en                                                                         
Hit http*//archive.ubuntu.com saucy-security/universe Translation-en                                                                 
Ign http*//archive.ubuntu.com saucy/main Translation-en_US                                                                           
Ign http*//archive.ubuntu.com saucy/multiverse Translation-en_US
Ign http*//archive.ubuntu.com saucy/restricted Translation-en_US
Ign http*//archive.ubuntu.com saucy/universe Translation-en_US
Ign http*//archive.ubuntu.com saucy-updates/main Translation-en_US
Ign http*//archive.ubuntu.com saucy-updates/multiverse Translation-en_US
Ign http*//archive.ubuntu.com saucy-updates/restricted Translation-en_US
Ign http*//archive.ubuntu.com saucy-updates/universe Translation-en_US
Ign http*//archive.ubuntu.com saucy-backports/main Translation-en_US
Ign http*//archive.ubuntu.com saucy-backports/multiverse Translation-en_US
Ign http*//archive.ubuntu.com saucy-backports/restricted Translation-en_US
Ign http*//archive.ubuntu.com saucy-backports/universe Translation-en_US
Ign http*//archive.ubuntu.com saucy-security/main Translation-en_US
Ign http*//archive.ubuntu.com saucy-security/multiverse Translation-en_US
Ign http*//archive.ubuntu.com saucy-security/restricted Translation-en_US
Ign http*//archive.ubuntu.com saucy-security/universe Translation-en_US
Fetched 342 kB in 20s (16,7 kB/s)
W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/main/source/Sources  404  Not Found

W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/source/source/Sources  404  Not Found

W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/main/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/source/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/main/binary-i386/Packages  404  Not Found

W: Failed to fetch http*//extras.ubuntu.com/ubuntu/dists/dists/saucy/source/binary-i386/Packages  404  Not Found

W: Failed to fetch http*//ppa.launchpad.net/tualatrix/ppa/ubuntu/dists/dists/saucy/main/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//ppa.launchpad.net/tualatrix/ppa/ubuntu/dists/dists/saucy/source/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//ppa.launchpad.net/tualatrix/ppa/ubuntu/dists/dists/saucy/main/binary-i386/Packages  404  Not Found

W: Failed to fetch http*//ppa.launchpad.net/tualatrix/ppa/ubuntu/dists/dists/saucy/source/binary-i386/Packages  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/saucy/source/Sources  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/partner/source/Sources  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/source/source/Sources  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/partner/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/source/binary-amd64/Packages  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/partner/binary-i386/Packages  404  Not Found

W: Failed to fetch http*//archive.canonical.com/ubuntu/dists/dists/saucy/source/binary-i386/Packages  404  Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.

A:

Failed to fetch http://extras.ubuntu.com/ubuntu/dists/dists/saucy/main/source/Sources

The repository doesn't have such path. You should look for an entry similar to:
deb http://extras.ubuntu.com/ubuntu/dists/ saucy main

Notice the end dists in the url. Remove all this lines. Or if you want you can use sed to fix the lines in one go.
sudo sed -i 's/\/dists//g' /etc/apt/sources.list /etc/apt/sources.list.d/*.list

