Sometimes, at the beginning of a game (especially if you are "moving up in class"), your mind set is that of a spectator, not an official and you may find yourself "watching the game as a fan", not as an official. You must just have the mental concentration and disipline to shake yourself out of it and start officiating.
Of course, this never happens to me
(some coaches would say it never happens to me because I never shake out of it and just observe the entire game)