I've been on a regional flight (don't recall the airline) where, after boarding, they made an announcement that they had to remove two people for weight reasons and were looking for volunteers . Funny money was offered, but given the next flight would have been a day or so later, there were no volunteers despite multiple requests. Then they switched tactics and said they would choose the persons who checked in last. They called two names, and no one came forward. (yeah, I wondered, too, why they didn't go talk to them at their assigned seats?!). After a couple of rounds of that, they said they would have to involve airport security (this was before the UAX vs. Dao dragging incident). That caused the one selected passenger to speak up and tell the FAs that he was the companion of the blind passenger next to him, and if he was removed, the blind person was not going to be able to travel. I think at that point some other passengers nearby volunteered, earned a cert and applause, and we finally took off.