When I was 14, I worked for a guy who owned his own business. One of his quirks was to give $2 bills and wheat pennies as change. Always. It was sort of a marketing gimmick- they see the $2 bill and remember exactly where they got it.
But, strangest I ever received was a $5 that someone had done some artwork on. It was expertly and masterfully done, but was so horribly inappropriate that I could never hand it to a cashier. It was obviously meant to offend anyone who saw it, but I just laughed. I shoved that thing in a self-checkout first chance I got.