spinning engines backwards

In rare scenarios where you let a car go forwards down a hill in reverse and pop the clutch, the engine will spin backwards and gain RPM but not actually turn on and let you give it throttle. How realistic is this and would/should this damage the engine?

 

Pretty realistic. The engine can spin in reverse, but it won't run because the intake/compression/combustion/exhaust strokes will be reversed. I've seen people mention that timing damage may occur, as some timing systems are designed to only run one way, but otherwise mechanically the engine can rotate backwards just fine.