Every time I use one of the new, harder to counterfeit 20 dollar bills, the clerk looks at me like I'm a counterfeiter, and writes on it with a special marker to make sure it's valid.

In retaliation, I have started carrying my own plain black marker and writing on any bills I receive as change. The ordinary black marker, of course, makes a mark on the bill, so I promptly hand it back and say, "This one's no good, gimme another one."

Meanwhile, any small bills I receive in change are immediately held up the light and tested for quality, while I spy the clerk suspiciously.