They can "un-use" a discount ticket, too.

I was once stuck on the runway at ORD and they had to take us back to the gate because the crew was "stale" after waiting over an hour. I called CO's Elite (ha!) line on my cell phone and asked to be re-booked (only one more flight out that evening). They said they couldn't- my ticket was "used". By the time I got off the plane and to the gate, the next flight was fully booked and I was stuck there for the night. I left the next morning.

Well, if my original ticket from the night before was irrevocably "used", I should have gotten miles for that flight as well as the AM flight, right? Well, I didn't. Somebody must have figured out how to reverse the "used" status.