In 1990 on a loaded touring bike doing a small climb into a town called "Cave-in-Rock" the rear derailleur went into the spokes and I fell over. I was covered with coal dust from coal trucks passing by and must have looked pretty funny. Got it straightened out and still have it on another frame.

Have also had the chainsuck from mud on the mtb and even on the road from worn drivetrain. That gets your attention, you downshift, start to spin it up, and it just stops.