I learned how to jam the checker robots by wadding bills up in my pockets and making sure they're slightly damp, preferably from crotch sweat. When you try to feed them into the machine they either wad up the gears, or the reeking sweat shorts out the robot. Either way, a human attendant has to open the machine and fumble about inside it until they fish out the crumpled $1 bill.

I felt a lot better after I did that.