Sunday evening I was flying ORD-MSP. The plane was an A320. The board said that first was checked in full. There were 5 people on the waitlist. I was in seat 1A. Seat 1B was still empty until right before the doors closed and then an FA (in uniform) took that seat. I thought that was odd...

The flight is very short, but if I had been in coach and saw that, I would not have been a happy camper...

Probably, when the person checked into 1B did not show, the GA just boarded the FA in his/her seat...