I've only ever had one ride ending failure, quite a while ago now, caused mainly by my lack of maintenance (I'm a reformed man, honest!).
I was on the return leg of my commute, entering the cycle path which creates a shortcut to my street. I normally go slow here as a courtesy to pedestrians. I changed down into my 1st gear on the cassette, although must have still had quite a bit of momentum from the road.
As I shifted down in to first, the badly adjusted rear derailleur went straight in to the spokes. But did the replaceable fail-safe hanger snap as it was designed to? Nope. As the derailleur went into the wheel the hanger stayed solid and the derailleur ripped a good group of spokes straight out of the rim, completely pringle-ing the rear wheel.
The wheel was a write-off, but amazingly the hanger and derailleur were fine. Luckily I only had to walk a couple of hundred feet to my house, as the rear wheel was so bent it was wouldn't turn past the seat stays.
Since then I've kept my rear derailleur perfectly aligned.
