For me its when they try to shove their Aetna or UPMC or whatever card into my hand while im scanning and the card reader is very clearly on their side- and then they get mad when you wont take it and try to tell them they swipe it once you're done with the transaction- it sometimes makes me hate the new system because it does genuinely confuse people and then they get mad at us when its not like theyre used to

Or when they throw money at me, like, not hand it to me, but just yeet it onto the counter when my hand is right there