It's probably happened to me a dozen times that I can actually remember. The first time I can remember was a $20 on the floor of a Blockbuster Video. Most recently I found a 50 euro note on the ground in La Reunion about a year ago. But I guess that's technically not a $20 bill, so maybe it doesn't count.

If you allow for faux currency exchange, keeping your eyes aimed down at a renaissance fairground is pretty much guaranteed to turn up enough food tickets for a turkey leg.