I was talking to a United Gate Agent I met at my dentist's office, and she said about the biggest no-no (actions that can get you in REAL hot water) there is is 1. Upgrading a free coach award, and 2. Double Upgrading (2 class upgrade...from Coach, skipping Business, and going into First on a 3 class plane) using upgrade certs. The only time they can get away with this is when Coach is oversold.