Man...there are so many.
There's the time I adjusted that guy's bike when I was working at a shop to help pay my way through college.. I was sure I'd attended to everything. But the guy was back later that day. He'd shifted the rear derailleur right into his rear-wheel. My boss was apoplectic. With good reason. Although to this day I'm not convinced it was my fault. it was a Simplex Criterium after all, and those were notorious for going out of adjustment if you looked at them too long.
then there was the time I slit a finger wide open on a wire bead...you know about that. Another job in the shop, working too fast, not being careful. I bled all over everything. The bead was exposed...Ugh.
I think my favorite, though, was when I was assembing a bike for a friend, more recently, and trying to get what was absolutely marked as a swiss bb fixed cup into a Mondia bb shell. I spent over an hour trying to get the damned thing in, constantly screwing and unscrewing...it was really tight. Finally, exhausted, I gave up... next day I come to find out I had bought one of those rare bb cup sets for worn or stripped bb shells...campagnolo made these slightly oversized. A lovely product. I just wish I had understood the cryptic stamping on the cup that would have cued me to what it was.
Charles Andrews
Los Angeles