I've seen loads of people bumped at the gate, I guess they bank on having some no-shows, even at the last second. I'm also suspicious that they give away the same seat number twice sometimes and then change it or op-up as the passenger boards if both passengers turn up.

I've been approached in the check-in queue and offered $150 if I was willing to take a later flight. It was a Friday and I was travelling on a fare which cost $2000 so you can imagine what my response was. The agent didn't care, he just wandered along to the next person.