Had a pretty funny one a few weeks ago. This was on a 767-300ER, where seat 1A is the dedicated crew rest.

Last guy on the plane (must have figured out from the gate agent activity or something that he was) came in and tried to sit in 1A. He was not even ticketed in Polaris I believe. He sat there and was really stubborn about moving. The FA had to inform him that due to the seat being a crew rest, the flight would not take off with any passenger in that seat, before he went to coach in a huff.

To my eye, that's borderline grounds for de-boarding, you know that passenger is going to make the FAs' lives miserable the entire flight.