I remeber once actually being downgraded! I was flying internally in the Us on United. I used my vouchers to upgrade. Got a nice seat in first class. Then about ten minutes before takeoff, I was asked if I could move back to business class. They told me that the seat was double booked. I had no choice but to do so. And who do you think got my seat? A united Cabin crew! Obviously business class was just not good enough for him.
This is one of the reasons I now give my business to BA.