A Match is a few mins into the second half when you realize that the teams did not change ends at half time.
I know that you would have to restart the second half again and nullify EVERYTHING AND ANYTHING which happened in the time which was played.
This raises a further question: What would happen if you sent somebody off in this now nullified time?
I say you would have to allow them back on.
This question was asked at a Referees Seminar in [another country[ last year and the organizers were dubious of my answer.
USSF answer (January 26, 2011):
Without having full details of what occurred in the match, and knowing full well that there is nothing in the Laws of the Game to cover this situation, our opinion (and it is no more than an opinion) would be to stop play as quickly as possible, have the teams switch ends of the field, and restart with a dropped ball where the ball was (but with this location determined by the reversed polarity of the field — i. e., if 3 yards past the midfield line into the Blue end of the field, drop the ball 3 yards past the midfield line into the NEW Blue end of the field). Allow all earlier actions to stand, regardless of what they were, and then provide full details and a “ea maxima culpa” in the match report. There is no basis in the Laws to restart the half anew; doing so would imply that the actions occurring since the original start of the second half did not occur and the culprit would thus escape without punishment.