At my usual grocery store you need to tell the cashier if you're using debit (exact or with cash back) or credit and then confirm the amount before scanning the card.
I think I scanned my card prematurely the first twelve times I shopped there because I've never seen another store do things in that order.
I once had a customer cancel her transaction and then she doubled over laughing hysterically. Instead of hitting 'accept' on the signature screen she hit 'clear' because she thought it meant that her signature was clear.
It's always nice when a customer gives you permission to laugh at their stupidity.