That I have no idea how either got into my house ? <| sep |> I only eat sandwiches . <| sep |> First saw this story about diagnosing machines in How Not To Program in C which I highly recommend . A system administrator for a major university was responsible for keeping several hundred DEC machines running . He quickly learned how to diagnose broken machines and figure out which board was bad . To get a steady supply of spare parts he had to purchase a service contract . This meant that a DEC service representative was supposed to come down , diagnose the machines , and figure out which board was bad and replace it . In practice , the university staff had strict orders to never let the DEC service representative near the machines . A typical service would start with the administrator telling DEC which board was bad . The service representative would come down , find the broken board on the administrator ' s desk , and replace it with a good one . No running diagnostics or other work , that was all done for him . A few years later , DEC instituted a Smart Spares program . The idea was that you had someone trained , on site , who could figure out which board was bad and order a replacement from DEC . Of course this suited the university , because it had been operating that way for years . The catch was that you had to take a class , at DEC , to learn how to diagnose systems . The system administrator jumped at this opportunity . He needed the vacation . He practically slept through the two days of classes . The third day was devoted to a lab . The instructor had set up three machines . The students were divided into groups and were supposed to spend the morning finding out what was wrong with their machine . Our hero turned on his machine , looked at the flashing lights for a minute , and then said , Bad disk card . The instructor was a little surprised . How do you know that ? Lights aren ' t right . The administrator moved to the next machine , looked at it , and said , Bad memory interface card . At the next machine , Bad processor card . Three machines down . This exercise was supposed to take three groups all morning , and this one guy had diagnosed all the machines in two minutes flat . I talked to him a while back , and he told me that if he knew he was being timed , he would have worked faster . Undaunted , the instructor moved over to the machine reserved for the afternoon problem . This one has been set up by field service and was supposed to have a really tough problem , next to impossible to find . The instructor knew the problem couldn ' t be identified by looking at the lights and waited to see what our hero would do . The guy opened the back , and even before he hit the On switch pointed to a board and said That board is bad chip U 18 . It ' ll cause an intermittent data bus parity error . Now the instructor knew that the guy was good , but spotting a bad board without even turning the machine on ? Impossible . How do you know it ' s bad ? he asked . The administrator pointed to a small blue label in the corner . See that dot ? I put it there so I ' d be sure that DEC didn ' t trade me back my own boards . This board is from the university . I ' m the one who originally found the problem and showed it to DEC field service . The morning and afternoon ' s problems now solved in the space of about 10 minutes , the class decided it was time to go out for pizza and beer .