This is going back several years, but I've never forgotten the incident.
Checking bag at DCA for an international itin. Guy saunters up to the premiere lane, wife and kid in stroller hanging back. The guy says, "We are checking in for the flight to Denver." The Agent looks at him, a bit puzzled, and says, "We don't have any flights to Denver, sir. Perhaps you have a connection?"
Guy immediately gets huffy, saying he knows where he's going, etc. So the agent asks for his itin, looks at it, and then says, "Sir, you're at the wrong airport. Your flight leaves from Dulles."
Guy proceeds to go ballistic, starts yelling at the agent, telling her that he's elite, their best customer, gives them hundreds of thousands in business every year, and by God she better get him to Denver." His wife looked like she wanted to get swallowed up by a giant crack in the earth, but this DYKWIA was not going to be deterred and continued his ranting and demands. I left before I could see the resolution.
I felt awful for the agent and was thisclose to saying, "Sir, you're the one who showed up at the wrong airport. Perhaps you should be a bit nicer." But was reminded of the incident a colleague had recently told me about. She was checking in in the Elite lane and a similarly entitled DYKWIA showed up super-late for an international itin, yelling for everyone to get out of their way and for the agents at the check-in desk to hold the plane for them. When my little old lady colleague (truly a little old lady, about 5 ft tall and so kind and mild mannered) looked at him and said "Perhaps you should have strived to arrive to the airport on time for your flight," this DYKWIA made a fist, cocked his hand, and barely refrained from belting her.
I truly feel for UA staff who not only have to deal with entitled DYKWIAs, but those who become enraged enough to be dangerous.