That's also a hidden benefit of version-control systems. Sometimes when a program starts to barf unexpectedly, and I know that I did something funky at two o'clock in the morning last night (when everyone writes their best code ... c'mon, admit it...), it's very handy to be able to side-by-side compare the two latest revisions.

A couple of minutes later you find it, slap your forehead “Doh!!” ... and grimace, once again reminding yourself not to do that when you have a six-pack hangover.