I was in the tournament Crabbok is talking about. It wasn't X-Wing, but it was a game that does require you to assemble a deck beforehand. And believe it or not, the officially posted tournament rules for this game make no mention of having to use the same deck throughout the tournament. Maybe because they assume it's such a common sense thing, they don't have to spell it out, but all the rules say is players must bring "any materials specifically required for a particular tournament format, such as assembled decks and/or deck lists." Someone actually had a printed copy of these rules, and was so sure this was stipulated, he was shocked to find it wasn't. So I didn't feel comfortable dropping the hammer on this guy and completely ousting him from the tournament. Jo-Jo beat me with his legal deck in the third round -- rather soundly too, and it was my sole loss on the day -- and while we were trying to figure out how to break three-way tie, some wanted Jo-Jo ousted altogether, some wanted to just move on. It was my suggestion to reduce him to third, on the basis that his legal deck won both games it played, and his illegal deck lost. No real competitive advantage was gained, but there had to be some kind of consequence. Not being able to play for the championship seemed to be the simplest solution.
I guess the main takeaway here is never assuming anything is a given, whether you're a TO or a player.