[colhom]

It's actually a valve and a piston you have to worry about. At normal engine speeds the valves will simply snap shut if the cam stops spinning.

Valves slapping piston is generally only a worry with an interference engine design.[1]

1. https://en.m.wikipedia.org/wiki/Interference_engine

[Tiksi]

Yeah, I was thinking of interference engines, especially OHC/OHV Interference designs. "Timing belt failure" is deeply anchored to Honda engines in my mind, which are pretty much all interference.

[Infernal]

My '08 Honda Fit may be interference, but it also has a timing chain that /should/ require no maintenance for the life of the car.