Software seems to be organic. It seems as though if something works perfectly well for years and is never touched that it will continue to work, but one day you do something you've done every day forever and it doesn't work. It's like it's caught a cold or something, and you have to interrupt whatever you're doing to find out why and fix it.
On Friday, as we were approaching the deadline for testing the hugely important new release of our flagship product, I discovered a bug in an important feature. Upon presenting it to the design engineer I heard the famous last words every test engineer has heard a million times: "I don't understand? "I swear it worked when I tested it!"
Swear all you like, kiddo. It's busted.