I thought of this question because someone joked about double-dipping their hands in the chocolate fountain at Golden Corral and boy did that invoke one of my least favorite paying-for-college memories.
Yes, someone did dip his hands into the chocolate fountain at the Golden Corral. Worse, he was a repeat offender, a man that was at least in his 30s if not older slurping it off of his fingers and all, sometimes while making eye contact with me or my coworkers. Worse, there was no enforced rule against doing so, at least at my location, so my manager just told me to let him do it, don't make a big deal out of it, and hope he doesn't bother anyone else.
That same manager once insisted on me making the place extra clean a little before Christmas, so they insisted that I use double the amount of cleaning bleach in the same bucket. I explained that's not how cleaning works or how OSHA compliance works. I got a write-up. I said that wasn't an offense that qualified for a write-up, and what they said was "thanks for the tip, I'll find something that is. Your word against mine." 
That same manager punched me out early without telling me, because the place wasn't perfect enough before I left over an hour late, missing my family waiting to pick me up outside by that long to go out to do holiday stuff. I did call that in on the supposedly anonymous tip line later, but you can guess what happens when an anonymous tip about wage theft is called in on a manager that already knows who would call in that tip in a "right to work" situation. 
That same manager was fired a week later for embezzlement, and not the cool kind. They were writing up and firing people for months for money missing from the register. I found out when collecting my last check and noticed someone new. 
No one incident comes to mind, but my last job saw me working in a lighting and furniture outlet. At least half of our customers were some manner of petty boug - homeowners, landlords, AirBnB owners, and the like.
They would come in to get furniture and fixtures for cheap, sometimes in bulk, which left us busting our backs and sweating our asses off in the un-air conditioned warehouse or out in the hot sun, loading hundreds or even thousands of pounds of merch into their expensive trucks, Land Rovers, or G wagons. Even the ones who rented transit vans or Uhauls tended to own Mercedes or BMWs as their personal cars. Oh, and Teslas. Lots and lots of Teslas. I saw more Teslas at that job than anywhere else in my life.
These people would almost never tip, and we weren't allowed to accept them even if they had, since parking lot service of that kind was company policy - it was expected that we would break our back lifting sofas and fountains and stone and marble tables and whatever else, so it would be inappropriate of us, they said, to accept gratuities (we did anyway, fuck em).
The worst, though, was how they always seemed to be looking down their noses at us. Talking down to us. Treating us like scum, like field hands. I knew even before I started that service work was inherently demeaning and undignified and soul destroying, but this job really made me feel it, made me viscerally understand in a way that only direct experience can. I hope I never work customer service again, but if I do, I know the hatred I felt for customers will come right back to me.