Used to work as an overnight grocery stocker/checker. I always seemed to get stuck with this one older (late 60s) lady, who was, quite honestly, a bitch. I learned to not say anything to her after getting sneered at for politely asking how her night was, and if she'd found everything okay. Another delightful quirk, was her total insistence that I not start running items through until she put EVERYTHING on the belt, and then go as slowly as possible so she could watch the screen ("I don't want you people ripping me off!").
I don't know what you folks consider fast, but to me, scanning one item per 10 seconds or so is slow af.
Run her items through at a snail's pace, and after I run a few yogurt cups through, she freaks, insisting 2 out of the 10 cups she had grabbed were the wrong price.
I do what any good cashier does, and get a coworker to run a price check on the 2 cups in question. While waiting, I make a move to finish running her items through and she gets even madder, thinking I'm trying to "rip her off some more", so I'm stuck there, waiting in awkward silence with this pissy old bat.
She absolutely loses her MIND, bitching the 2 of us out, accusing us of being liars and thieves, and how the store is ripping her off, and how she's never coming back here again, blah blah blah...
Thankfully, the overnight manager had just come up front and heard this bat chewing us out. He asked what the problem was, coworker and I explained the situation before the pissy old bat could.
Manager took one look at her. "Ma'am, if that's how you treat my employees, I don't think we want your business. There's the door."
She threatened to call the store manager in the morning and stormed out without her groceries. We never saw her again.
All that drama over 10¢.