What kind of customer pisses you off the most?

Today I had a customer that bought a vitamin b-12, 30mL liquid in drops.

She paid with debit and i handed her the terminal and she punched in her pin and pressed okay. She then said 'oh nevermind i dont want it' with the transaction already made.

Normally we only give out store credits, but i figured if this woman was this stupid, she would probably not be worth my time explaining how the store credit works. I just refunded the money and she left.

I fucking hate people.