Ally McCoist, who plays a former Celtic player is, in real life, a former Rangers player (the Celtic-Rangers rivalry is emphasized in the film). In the stock footage in which they show McCoist in his glory days, they actually super-impose Celtic colors on to his blue Rangers shirts. Even though, in the movie, he is wearing green and white, the design of the kit is obviously that of Glasgow Rangers.
Didier Agathe plays for the "baddie" team Rangers in the film. In real life he plays for their rivals Celtic.
Ally McCoist wanted the team to win at the end of the film to be Rangers and not Celtic, to make Rangers look the better side, in doing so, the script changed the teams around, making his part in the film that of a former Celtic player, which he also wasn't happy about.