I was once in Pasco, Washington for a 6am jumpseat to work in SLC. The flight was full, but this gate agent actually let the plane depart with an empty jumpseat (my company's plane, my company's gate agent).

Ok, not a huge deal, another plane in a few hours. So, I could tell that things were going to be squirrelly for this flight, too, as it was oversold.

So, I called crew scheduling and told them the story so far, and that if this gate agent didn't let me on this plane, I might not make it to work. Then I asked them to call the plane (ACARS or cell phone to pilot) and tell them that I was waiting to get on board.

It worked out, but literally at the very last minute. Crew scheduling called the gate and told them I should be put on board.

I filed a complaint with the company over this gate agent, and the response was that this had happened before and that it would be dealt with.