A substitute, warming up behind his own goal, enters the field of play, touches the ball and tries to prevent the ball entering the goal with his foot. The ball, however, enters the goal.

What action does the referee take?

USSF answer (August 23, 2011):
The referee should play the advantage and award the goal. The referee should then caution the substitute for unsporting behavior for entering the field of play without the referee’s permission, including all details in the match report. (The referee could also consider a second caution for unsporting behavior for interfering with play and thus send off the substitute for the second caution in a match.) Finally, the referee should prevent substitutes from warming up behind the goals. However, in some stadiums warm-ups are allowed behind the goal (because there is no space along the touchlines).