Pittsburgh entered week eleven with starters having missed 31&#160;games due to injuries. The San Diego Chargers scored first, and led 7-0 after the first quarter of play. On the second play of the second quarter, the Steelers' James Harrison forced a fumble in the endzone, then forced a safety by tackling Marcus McNeill after he had recovered the fumble. Jeff Reed converted on a field goal as time expired on the first half, pulling the Steelers within two points. Reed kicked his second field goal of the game from 41-yards to give the Steelers the lead going into the final quarter. The Chargers drove 74&#160;yards on 17&#160;plays, converting on a field goal to give them the lead again, with 6:45 remaining. On the Steelers' ensuing drive, the offense drove 73&#160;yards and Reed converted on his third field goal of the day, giving the Steelers an 11-10 lead with 15&#160;seconds remaining in regulation. The Chargers attempted to score on their final play, but Troy Polamalu forced and recovered a fumble, returning it for a touchdown. The call was reversed when the officials ruled that San Diego had made an illegal forward pass. After the game, head referee Scott Green admitted that the touchdown should have been counted. With the touchdown not counted the final score kept Pittsburgh under the 5&#160;point spread by which they were favored, and resulted in over $32&#160;million being lost in bets. The game was the first regular season game in NFL history to end with a final score of 11-10.
Question: Which penalty called back a touchdown for Pittsburgh?
Answer: illegal forward pass