So one night, I go into a supermarket for the beer. I get two six-packs of Michelob. (Hey, my beer tastes have changed, and this was well before the current renaissance in American beers.) I go to the express lane. In line in front of me is a guy who also has two six-packs of Michelob.
The cashier rings it up, and it came to something like $3.98. The guy hands her $4, but he evidently didn't care about the two cents change, because he just picked up his beer and walked out while the cashier had her back turned. I pick up my beer and plonk it down on the counter. The cashier turns around, smiles at me, hands me his change, with receipt, puts my beer in a bag, and says "Thank you, come again!"
What else could I do but say "No, thank -you-!", pick up the beer, and walk out. All the way to the door, I kept waiting for someone to say "Hey you! Wait a second!" but it never happened.
Ever since then, when I've gotten too much change back (it happens sometimes), I've given it back. I figured that there's no way that the free beer plus change back can be topped, so I'm not even going to try.