I doubt that this will happen to anyone in my lifetime, but how about a stone piercing the condenser, right next to the engine intake? That might be fun...

As for 134a being flammable, well, yes it is, but you'll never notice. It cannot burn unless it is combined with air at higher than ambient pressure. The oil is another story entirely. And refrigerant hitting other burning stuff in the car could break down into some rather nasty chemicals.

The potential for problems with HC refrigerants are not in the condenser. A leak there is unlikely to cause trouble, even in a wreck. There is so little of the stuff in there. The problem is if the evaporator leaks and propane mixes in the air inside the car, potentially causing an explosive atmosphere. There's a video online showing some HC sellers showing how safe it is -- until they do explode the car, bending the window frames out. Evaporator leaks are not exactly rare.