Some people recommended deleting your ~Library/Preferences/com.apple.finder.plist file to fix it.  I wasn't able to simply delete the file to fix the problem.  I had to go into my back up account and make a copy of the file from ~/Library/Preferences/com.apple.finder.plist and then move it into my user account.  I'm sure you could do the same thing with your Guest user account, too.

For me, the problem was caused by making the text in QuickLook copyable.  I noticed when I turned on defaults write -g QLEnableTextSelection -bool true that the blank Quicklook bug was coming up as talked about in this thread.  When I turned it off, (defaults write -g QLEnableTextSelection -bool false) the problem went away.

Quicklook is broken in Lion
Are there some living things where the distinction between plant and animal is blurry or unclear?  I wouldn't know where to put something like yeast.  I also thought of photosynthesis, but then I heard that some salamanders have photosynthetic cells.  So, what's the difference between a plant and an animal, formally?  
