Just like the course of match. This applies to supporters joining the game too. A substitution or a new supporter joining the game seems to have an immediate impact on the course of the game as if the game engine temporarily 'goes mad'. Sometimes the effect can be very negative, causing you to lose the match. Some of these things have been happening way to frequently and evidently, and yet some people denied it.
Now there is a new thing in the game. Since a few days, it seems that goals are being scored for fun. Players are going through the defense way too easily and scoring past the goalkeeper.