The one I share every time this topic comes up, from Lore Sjöberg:
A fifth-level paladin drives his car to the repair shop.

He gets out and says to the mechanic "It's really weird. Normally I fight for justice and righteousness, but every time I get in this car I have this incredible urge to run over old ladies, drive way past the speed limit, and pick up hitchhiking demons. Can you help me?"

The mechanic looks the car over and says "Yeah, I see what the problem is. Your alignment's off."