I've got a small roulette wheel in my game room. It's got two balls in it. Everyone that plays is required to choose a number. Whenever a ball is 'stuck' and the machine needs to be wiggled a bit to get it out, we spin the wheel. If the wheel lands on your number, you go to the fridge, grab a beer, and finish it ... immediately.
I've got a Skateball that likes to get balls stuck under its upper flippers and on top of the upper lanes. Plus the playfield is blown out to the wood in spots, so it's really dirty. I've also got a SFII that get's balls stuck in the Ken/Ryu and Zangief loops every once in a while.
Also, balls that get stuck but come back during a 'ball search' are counted as balls earning a spin of the wheel.
Needless to say, it's not everyone's favorite rule.