I had a person who bought clearance x-mas tags. Two sets. She goes through her receipt huffing, "Are you ****** stupid? Do I need to watch you scan my stuff? I didn't have two in my basket!"
Without a word, I go through her bag, pull out TWO items. Said nothing. my line of customers smirk "Dumbass" on her. And she was flustered and obviously embarrassed but decided to abuse me further. " Get your manager. I'm done with you."
And then what happened?
Come on you can't leave us hanging in suspense

I know from experience that the problem with managers is they can reward a customer for bad behaviour.
Did care billing for a cellphone carrier for 11 months and knew that company policy was that charges not challenged withing 90 days were deemed as accepted.
Had one customer who'd been incorrectly charged for text messages - 6 months previously so I rightly refused a credit. The rules are there on the bill and had she called in the 90 day time frame I'd have no problem crediting. She then asked for a manager transfer and which she got, and a credit for the messages.
And my take away from the 11 months was that the corporations are bad but the customers are just as bad.