That's what happened to me at two different stores a week ago -- I received currency from two separate countries. What an odd occurrence!
A penny from the Bahamas, and a penny from Canada.
I return change/bills when I'm given back too much change. Under normal circumstances, I correct the cashier when I'm not given enough change, but I happen to collect coins & bills from other countries, so these two pennies are keepers. Both pennies are not valid U.S. currency, so it wouldn't do me any good to try to pass them off at the next store anyhow!
The penny from the Bahamas was particularly interesting - it has a starfish on the "heads" side!
How about you? What other-country currency have you received?