Some from work.
1) When someone plops their stuff on the counter and immediately start to swipe their Credit/ debit card through the machine, then repeat when the thing does 'nothing.' It goes something like this...
*swipe*
deleep (from the computer)
Customer: "It didn't do anything!11!!"
*swipe*
deleep
Customer: "Still nothing"
*swipe*
deleep
Me: STOP IT!! I have to actually log on first. I will tell you when to swipe.
Then at the end of the transaction, they ask "Am I going to be double (or triple or how many times I swiped my card) charged??"
Me: :banghead:
2) People who hold up the line to dig all through their purse/pockets/ friend's pockets and purses/ diaper bags/ whatever to find exact change.
2b) People who find exact change after they gave me bills, I have made change and closed out the sale...receipt in hand.
3) Slobs. From people who leave their trash everywhere to those who leave a massive pile of twisted clothes and hangers in the fitting rooms. Didn't your mama teach you right? My mother would raise from the dead to smack me upside the head if I did such a thing.
Most of my customers are nice and make my job great. The sucky ones, OTOH....well...they just suck! I keep telling my manager that one day I am just going to go postal and whack someone with a curtain rod.

layball: