I was having an argument with a referee friend and the question at hand was: if the second last defender of red team is lying on the field due to an injury or slipping(legs closer to his goal line, if it makes any difference) and an attacker from the blue team receives the ball from his team-mate being behind the third-last defender but not after the second-last which is still lying on the ground, is it an off-side? He said it would be, because the defender on the grass is injured so he does not count.

USSF answer (January 27, 2012):
The Law does not discriminate between players on their feet and those on the ground. The defender lying on the ground would count as one of the opposing players in the offside situation. However, if the player who received the pass was not beyond the second-last opponent, then he was not offside.