I thought this was funny (or maybe sad). Here is an excerpt from a Fox News article, syndicated from Military.com, on the tradition of the Army-Navy game posted on TexAgs.com. This is under the heading Lesser Known Traditions.

At West Point, there is a tradition of adding a "12th man" to the team. The tradition began in 1922. Afraid he might not have enough players to finish the game, the coach remembered that there was a Cadet in the press box helping to identifying players during the game. His name was E. King Gill and he was suited up and told to stand on the sidelines just in case he might be needed.

He never played in the game, but the notion of being ready to help your team was adopted by the student section and over time the entire student section was dubbed "The 12th Man." Additionally, the conductor of the pep band will paint his face with the number "12."