Originally Posted by OutOfOffice
Also, I've read threads in the past on various airline forums that basically indicate that you are more likely to be refused check in when the flight is full and they want to minimize the risk of an oversell situation.
That only makes sense...they have to offer money to people to not board, or deal with the hassle of IDB stuff if they can't get volunteers. From their standpoint, you've essentially volunteered to not get on that flight for no compensation. They're not going to go out of their way to help you out if the flight is full.