The referees decisions (or lack there of) can decide the outcome of a game. This have become more evident as the season progresses while watching the MLS games following the exploits of our New England Revolution. The Revs have had their share of injuries, resulting in loosing six "front-line" players for several periods; a couple of them basically for the entire season. So while playing under stress, it doesn't help to get calls against you. In those moments it seems like the opponent has an extra person on the field.
Perceptions created during the game can be incorrect. How so? The US Soccer Association has a weekly review of the referees decisions. They will highlight several instances of either good calls or bad calls for instructional purposes. With video clips, diagrams, and rules of the game to reference, they will walk through a game situation step by step to show how and why the correct call should have been made for this situation.
The most recent week is reviewed here. I find it great to see the details laid out like this. It really helps to learn the game and hence be a better informed fan of the game and team!