At some stores, the clerks can enter the amount at the register. I doubt the clerks would just have their buddy waiting conveniently to hold the $ though. They probably signal the person and their buddy gets in their line then.
I check my receipts to make sure nothing was rang too many times. That's an easy mistake and I don't have the $ to pay for stuff I don't get.
I've heard swiping the card at gas pumps can make it easier for scammers. They say to just pay inside to get around it.