They can't do basic math. I filled up with gas a couple of weeks ago, and the pump price was $.02 higher than the sign. I went into the counter to inform them of the mistake. The kid was trying to figure out how much I was overcharged.
I looked at the receipt and noticed I had put 15 gallons in my tank and told him it was $.30, while he was using his phone to calculate the total. After about three minutes, he said "You are right", and gave me $.30.
I frequently give extra cash when paying to get a five or ten back. I might pay $22.50 on $17.50.
Half the people I do this with try to give me $2.50 right away telling me I gave too much. I tell them to punch it into the computer and see what happens. They are amazed at the result as if it is magic.