Hello, my question is while the ball is kicked and pass the goalkeeper and headed to goal, at this moment if a fan inters the pitch and blocks the ball from passing the line, what will be the referees decisions?

USSF answer (November 14, 2011):
The fan is considered to be an outside agent. If an outside agent enters the field of play the referee must stop play (although not immediately if the outside agent does not interfere with play), have the outside agent removed from the field of play and its immediate surroundings, and then restart play with a dropped ball in the position where the ball was at the time when the match was stopped, unless the ball was stopped inside the goal area, in which case the referee drops the ball on the goal area line parallel to the goal line at the point nearest to where the ball was when play was stopped.