In the prologue of the Tour of Switzerland today, Andy dropped his chain and had to finish on his road bike. I replayed it in slow motion a few times and it looked like he was shifting his to his small chainring for a climb and hit a small bump at the same time.
Later, Cancellara possibly dropped his chain (yes I think he was shifting, but to his large chainring) and luckily got his back on right away. He ended up climbing in the big ring, fully cross-chained, which I assume was because he didn't trust the front mech.
Poor technique, bad equipment, or soon-to-be unemployed mechanics?