This was a first for me.
I was working on a bike a few weeks ago that a guy had brought to us while set up outside of a community center. He had a good looking bike, and he said he really only needed a little tune-up, maybe some break adjustment, and that his gears were not shifting completely, so he wasn't able to shift into the highest gear. I threw the bike up on the stand and began working on it, and after some tinkering with the rear derailleur, I got the gears to shift properly. Deciding to check one last time that my work was done, I shifted from one gear to the next, watching the chain jump up; then I hear a "snap" and realize I have broken the shifter before even looking at it. I knew it was a big problem; I didn't have one to replace it, and when the plastic breaks on these things, there is no putting it back together.
I explained to the guy what happened, apologize profusely and feeling like a real idiot, because, for the first time, I had manged to makes someones bike WORSE than how they brought it to me. At least before the worst I had done was not being able to improve it. And I forgot to bring a bike pump to this event. Needless to say, I did not pat myself on the back at the end of the day.