I've learned things being a cashier: People don't like being told their coupons don't match what they are buying or they are expired, they think we are a bank and always blame we cashiers if their cards don't go through, people think I either have very long arms or I am a Jedi: They put their stuff at the end of the register where I can't reach it (this is the speedy lane, where there is no conveyor belt), speaking of the speed lane, people are supposed to have 20 items or less, but 1 out of 5 people bring more than 20 items. Just blowing off a little steam.
People tell the same jokes over and over: While I am checking a big bill, they say "I just made that today", when an item is not scanning well, they say "it's free!" Those jokes weren't even funny the first time...