Most of the time I just suddenly don't have a seat (Seat Assignment: N/A)

Twice I've gotten a supervisor on the phone who could only offer this explanation:

"I see you were assigned 15A, but I don't see where it was dropped."

Uh, ok.

It's really annoying in the cases where upper deck is then full.

(For the record, most of these flights were SIN-HKG-SFO or vice versa)