The 12th Man

The tradition began in Dallas on January 2, 1922, at the Dixie Classic, the forerunner of the Cotton Bowl Classic. A&M played defending national champion Centre College in the first post-season game in the southwest. In this hard fought game, which produced national publicity, an underdog Aggie team was slowly defeating a team which had allowed fewer than 6 points per game. The first half produced so many injuries for A&M that Coach D. X. Bible feared he wouldn’t have enough men to finish the game. At that moment, he called into the Aggie section of the stands for E. King Gill, a student who had left football after the regular season to play basketball. Gill, who was spotting players for a Waco newspaper and was not in football uniform, donned the uniform of injured player Heine Weir and stood on the sidelines to await his turn. Although he did not actually play in the game, his readiness to play symbolized the willingness of all Aggies to support their team.

http://en.wikipedia.org/wiki/Texas_a%26m_traditions