I love it when they try the "I paid with a" higher bill. As soon as you offer to count it to prove it, they freak out and back peddle.
I had someone do that to me in the middle of a lunch rush at Clown In The Box. They claimed they paid a small order with a $20.
At that time we were required to drop *all* $20 bills, so it was pretty easy to prove, thankfully.