Yes, I had a boarding pass with the seat assignment but the FA was of the opinion that because I had boarded so late they had released my seat to the other guy. This of course was untrue because I had been at the gate for a while being subject to the usual secondary screening, otherwise I would likely have boarded soon after the rest of the First Class passengers.

Again, when the FA said "ok, go take an empty seat in coach - we have to leave now" there is no room for argument, whether or not you are in the right. You have to comply with a crewmember instructions or else you wind up in trouble. Such is the reality.

I've had other similar incidents too (including the now infamous run-in with Captain Daniel Broderdorf of Continental and it's follow up), all of which contributed to my final decision to relocate to a country that does not condone such institutional discrimination under the guise of security.