I actually had CO take away my first class seat once. It was a mileage upgrade EWR-LAX (on a 767), using paper certificates.
I left the certs at home, called CO on the way to the airport (from work) to see if they would be able to deduct the miles directly from my account at the airport instead. Agent said no, but she could do it now. I said never mind, I'll swing by my home to pick up the certs.

Get to the airport, and lo and behold! no FC seat for me. I was told that I was downgraded because I didn't have the certificate. I proceeded to wave it in front of the check-in counter agent's face and asked him to repeat what he just said. (I'm exaggerating).

Anyway, I begged and pleaded nicely with the gate agent, she put me at the top of the waitlist, there was one no-show, and I got my seat. But that was a pure stroke of luck.
If I had to sit in the back, you can be sure there would have been hell to pay...