Cassette Lock Ring Fail
I ride an 11-speed Prestacycle cassette which comes with a lock ring that does not have a crush spacer. About a year ago, during a long and fast (for me) descent, my crank locked up and I could neither pedal nor backpedal. I thought I was going to crash, but luckily the rear wheel could still spin relative to the freehub, and I managed to unclip and stop. Turns out the lock ring had worked itself loose and got wedged against a bolt securing the RD hanger to frame, locking up the freehub against that bolt. Replaced that lock ring with a Shimano Ultegra one and problem has never come back.