At a busy airport, up to a dozen aircraft waiting to land may fly the same holding pattern at the same time, separated vertically by 1,000ft. — a stack. Some ATC towers have dedicated Stack Controllers.
It's all very routine, but have there ever been any instances of incidents or accidents in stacks?
To the layman, it might seem like stacks could be hazardous — e.g. what if a plane in the middle of a stack lost an engine?