So this happened a while back when I was still fairly new to all this, but I like telling the story so here goes.
I was on my lane like any other day. Giving my pre-recorded "Happy Associate" speech that we all have. Nothing special. My store has these Coupon printers next to the receipt printers, and if you scan your shoppers card mid order, the coupons will print off as your things are scanned. This is never a problem, because most customers either don't care about the coupons, or are going to stuff them in their purse and go through them when they get home. If they are to ask for them mid-order though, I'm supposed to hand them to them, but I can't scan freshly printed ones until their next order.
So, this one lady comes through my lane with two things: A cartload of groceries, and a handful of coupons. I give her my speech, start scanning her things, and don't think much of anything about who or what I just brought upon myself. Before I can really react, she reaches around my scanner and scans her card herself. Then, the coupons begin to print. She isn't paying any attention to anything but those little slips of paper. Suddenly, a red, 15% off, Hot Pockets coupon prints out and rests next to me. She jumps a little.
Her: "Hey, can you hand me those Coupons?"
Me: "No Problem! Here you are"
She shuffles through them and picks out our aforementioned Hot Pockets coupon.
Her: "Scan this one."
Me: "Haha, I wish I could but I'm sorry. Those are for your next transaction. It's against store policy for me to use them now."
Her, very firmly: "Use it."
Me, caught off guard: "I really wish I could bu-"
Her: "IT'S MY COUPON. WHY CAN'T YOU!?"
Me: "The reason is that this transaction hasn't gone through yet. Since you're a plus customer you get these discounts based on what you buy previously. It would be like if a Buy One Get One printed off after scanning two of these, I couldn't just give you a free one since you haven't paid for the first two yet."
This was not enough.
She's getting red in the face, and asks for the Coupon back. I oblige. She then reaches around and scans it for me, thinking it will work. Coupons have to be scanned at the end, so all it does is bring up an error message that says "Coupon scanned, take total".
I don't say a word though, I just let her think it happened and send her on her way after paying. She then goes right to Customer Service and demands that I be pulled off my lane and brought over for mistreating her. I'm friends with the guy back there and he knows I'm pretty much the least likely person on earth to badmouth a customer, so he tells her I'll be taken care of and sends her on her way.
A days worth of Customers and Carnage later... I've received an online complaint, rated highly unsatisfied, and I'm a bagger for a week.
All in a day's work.