Had a rear deraileur fall apart. Oddly enough I was just returning from a Road 2 class and was within blocks of my home when the RD just fell apart. It had been shifting rather sloppy so I was paying particular attention at the moment, and saw the pieces fall off. It was the bolt through the spring... the back of the bolt, a large round nut had worked loose. Once I walked it home and put the tension back on the RD it all slipped right back together.

Oh, long before that, I had a Bottom Bracket spindle break in half... different bike. It really shocked me as it was a Phil Wood BB. I had just jumped on the bike and was on my way to work and suddenly both legs are at the bottom of the stroke. (glad I wasn't out of the saddle at that moment) I stood there over the top tube for a minute, trying to fathom what could have happened. I was only a few blocks from home so I walked back, and later tore into the BB to find the spindle broken clean in half. It was hollow. I sent to Phil and they sent me back a new one... but this new one was solid.