I had an intresting experience last week. I was traveling from BKK-NRT and was given a a Business Class boarding pass (upgrade had been requested using an SWU and had been in for a long time). After getting settled, being served champagne, etc--one of the GAs came and handed me back my economy boarding pass and said that another passenger had shown up (I'm assuming the best, a paid C that was late on a transfer).

Does this happen often? It was disappointing but I still got E+ aisle, so it was the same that I had reserved.

More frustrating was the 1 hour late for luggage to arrive ar ORD later and missing a flight to ANC where I had a confirmed upgrade (no one really cared on the International side)