I've had it happen to me before, but it has been a while. Once, I was first on the UG list with one person not checked in yet. Turns out he was at the gate but did not recheck in so the computer thought he was a no show and I was about to get this seat. Since he had his BP he "proved" he had checked in so he kept his seat and I remained back in main. The other time I just re-checked in to get my wallet BP to update.