NCAA: Illegal Substitution

[jTheUmp]

Two other ways it can happen:
1) substitute comes onto the field, communicates with a teammate (thereby becoming a player) and then leaving the field before the snap.

2) Player goes to the sideline believing that he's been replaced by a substitute. Then he realizes that he hasn't actually been replaced, and he returns to the field without sitting out a play.

Rules references: 2-27-9, 3-5-2d, and AR 3-5-2-III.