Originally Posted by
eigenvector
In this case it's important whether your "about 20 mins" was 21 or 19. If you arrived at T-21, with a valid BP and ready to board and the flight had been closed, then you are due compensation as someone involuntarily offloaded from the flight. If it was T-19, then it is your mistake.
But if the OP had showed at T-21, how exactly would you prove that after the fact? You can file a complaint and say you're due compensation, but ultimately it's a matter of whether the airline choose to believe you. No one is going to be testifying under oath.
I guess if this happens in the future, the thing to do is to take a photo of the closed gate IMMEDIATELY so you have a time-stamped picture that shows you were unfairly denied boarding. But even that's a bit of a stretch. Better to just show up earlier if possible.