While I stood waiting, I noticed this one guy who had been dealing with one of the clerks. I'll call him Mr. Angry, because he obviously was. He walked away from the counter towards the manager's office, and he pointed a finger back at the clerk (whom I'll call Sue) and said something I couldn't quite hear; it sounded like either "You need to be terminated" or "You're gonna need an attorney."
Mr. Angry went into the manager's office, and I could vaguely make out bits of conversation over the next few minutes; he was apparently upset at being moved from clerk to clerk, and he accused Sue of giving him attitude.
A few minutes later, Mr. Angry came BURSTING out of the manager's office and SLAMMED the door as hard as he could! Everybody gave him a wide berth as he stormed off. Before he left the room, he stabbed a finger in the air in Sue's direction and said, "I'M GONNA SUE YOU!"
The clerks and customers chatted among themselves a bit, speculating on what the hell THAT was all about. Sue asked for the next person in line, cheerfully saying, "You're my last customer before I get sued!" She clearly wasn't worried about him making good on his threat, although another customer warned her that Mr. Angry might, just might, come back with a loaded weapon. Better to err on the side of caution, he said. She wasn't too worried about that either, and neither was I. Mr. Angry seemed more like the big blustery all-bark-and-no-bite type rather than the violent-sociopath type.
Dude was probably upset about some screwup costing him the rest of his lunch hour, in which case I figure he'd just speed back to work as fast as he could, rather than coming back to play Terminator.