It’s good to watch out for convenience store clerks themselves, sometimes. Give them a twenty, they give change back for a ten. It happens too often not to be intentional. I’ve gotten so I watch where they put the bill, and tell them “here’s a twenty” beforehand. I’ve had a few that lied and denied it, insisting they got a ten. So, scammers all around.
Short change artists, back in the day before electronic cash registers, were more common than they are now. Some of those guys were really good at screwing the help up, especially the younger ones.