Have you ever noticed that some people will wait to the very last minute in the checkout line before counting out their change?
It's usually an older woman, buying a single item, and then pulling out her purse at the last minute, opening the change wallet, pulling out a pile of coins, and then sorting through them to count up to the right amount.
Could you not start this process a little sooner? You've now got a huge line of people behind you. All watching - all counting along with you. No pressure. Even the cashier is annoyed with you now. Come on, let's go! Writing a cheque would have been faster!
Then, to help the cashier, they'll try to hand over just the right mount of change so that they get a bill back, only they'll go the and do it the opposite way, so that they end up getting more change than what they started with. Annoying.