A quick anecdote to reinforce some of the points already made.
My parents were trying to non-rev to Las Vegas on a busy weekend. Almost all of the flight were oversold coming out of CLT, and they were at least 12-14 down the list to get on the first flight of the morning. My dad is the ultimate nice guy/schmoozer, and everyone loves him. He and the gate agent became fast friends. Anyways, the plane started to push and my parents were not on the airplane. They were still in the chairs and were waiting to walk to the next gate to try again, and the rest of the non-revs left the gate area. A few minutes later, the gate agent came and got them from the seats and said that they could get on if they really hurried. Evidently there were some really drunk girls on the airplane as part of a bachelorette party on their way to Vegas. As soon as the airplane started pushing back, they started screaming/celebrating and popped open bottles of alcohol that they had. The flight attendant called the captain, they stopped the push and pulled back to the gate. All of the girls got thrown off, and they let my parents get on.