My best: Working at Subway, as a sandwich slave. It was in the food court of a mall and got really, really busy around lunch hour on weekdays from local offices. This was a weekday; it finally slowed down around 2:30, and I was in front cleaning the counters and such. A customer came up with the remnants - less than half - of a previously-purchased sandwich. Without waiting for me to say anything, he threw it down over the counter, at my feet. “There aren’t words to describe how fucking awful this is! What the fuck gives you the right to serve shit like this to your paying customers?”
Now, I’d served somewhere around a hundred, hundred fifty customers in the past hour or so. I have no idea if this guy was amongst them. A glance at the spatter around my feet tells me it was a meatball sandwich, but for all I know, he could have gotten it from another Subway down the street. “I’m sorry, sir - I can go and get the manager from the back, and he’ll refund your money.” I start to do so, and the man screams, literally, “Don’t you dare fucking walk away from me!”
Luckily, my manager (who was actually a dumber-than-rocks prick, but that’s another issue) heard that from the back, and came out to deal with it. He gave the guy his money back on the condition that he didn’t ever return.