I changed the pulleys on my derailleur last night. This morning, I was on my commute and I hear the all-too familiar bolts and washers pinging as they litter the ground. I hopped off the bike and found that I had not tightened the bolts holding the plates down so I had a bunch of bolts, washers, pulleys, and a derailleur plate on the ground. Luckily, they all stayed in roughly the same area, so I (repeat: luckily!) found everything and put it back together. The last time something like this happened was when I started screwing in a pedal by hand and forgot to finish it off with my pedal wrench. It was only about 1 thread in, so when I hopped on the bike, it stripped the first thread. Plenty of threads left, so the crank was still usable.
I got passed by about 10 people while i was putting everything together and the only person who asked me if I was alright was an astronaut looking fellow on a carbon space bike. They're not all bad!
I'd love to know your similar stories