A few years back was instructing in a C-172 (at night). 400' AGL after takeoff I hear a loud bang and lose alot of power, and starts running so rough feels like it's about shake right off the engine mount (I lost a cylinder obviously) so I had to reduce the power even more to bring the vibrations to a tolerable level. Did a 180 and landed the opposite direction, not too much fun as it had to be done at 400' as there was no power to climb, ugh.

Turns out a renter before me liked to lean the mixture beyond peak EGT on the lean side for all operations, including full power takeoffs at sea level in cold temperature, sheer genius. So after frying some rings, I get it and it proceeds to suck a valve right through the cylinder head. The sight of it was amazing, it must really have pulled that valve through the head with some force, that was some seriously bent metal. Apparently the mechanic said the shape of the other cylinders were pretty bad too, so I guess I was lucky the entire engine didn't explode.

Don't trust rental airplanes.