My first job was as a cashier. There's a reason self checkouts are gaining popularity, a big part of it is people can be downright nasty to cashiers. I have a handful of stories from 9 years ago that still stick in my mind of people that we're just awful.
Customer: "I have multiple coupons"
Me: "Sorry, 1 coupon per person per day."
C: "Can you do separate transactions?"
M: "I'm sorry, but it's one per day."
C: "That's crap! I want to speak to your manager! Your customer service is terrible!"
M: "She's in that door down there. Go knock and she's going tell you the exact same thing."
Or
"The sign said this was 40% off!"
"Alright, let me get a price check real quick for you."
*coworker brings sale sign from opposite side of the store*
"I'm sorry, someone must have put that item down in the wrong spot. That sale sign is for paper, not pencils."
"Well can't you give it to me anyway? That's false advertising!"