Here's a webcomic for you, then
A similar cashiering annoyance was when people's items might get mixed up on the conveyor and I would accidentally ring up an item of the next person in line, which would lead to this exchange
1st person: "That's not my item, that's theirs. *pointing to 2nd person*"
2nd person: "Oh that's okay, they can pay for it!"
Hahaha...