This isn't a really a bug, but it's a funny little story...
I was working on some smooth scrolling for one of the games I have in development. One day I made big changes-- maybe 2 hours of work without a compile, just listening to music and coding. When I finished, horrors! The formerly smooth scrolling now had an irregular stutter that looked, for all the world, like a floating point error.
Over the next couple hours, I slowly tore out everything I had just added, trying to find out where the stutter had been introduced. I finally got it back to where I was before I started... and still a stutter! Figuring I broke something fundamental, I used a backup version and... still stuttering! Utterly terrified now that I had some basic problem with the smoothing routines that just became apparent--and those routines had been a monster to get "just right"-- I shut myself in my office, closed all the blinds, took off the headphones, and, to prepare for some hard-cure fury debugging... turned off Comcast Rhapsody so I could concentrate.