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2) Player goes to the sideline believing that he's been replaced by a substitute.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.

[Texas Aggie]

so no illegal participation foul supersedes the illegal substitution foul.

There is no foul for illegal participation.

A 12th player coming on, staying on, or not getting off is an illegal substitution foul. If discovered before or simultaneous with the snap, it should be blown dead. Otherwise, its a live ball foul. While it doesn't matter whether one participates in the play, a substitute is defined, in part as...

A legal incoming substitute becomes a player when he enters the field of play or end zones and communicates with a teammate or an official, enters the huddle, is positioned in an offensive or a defensive formation, OR participates in a play.

(note the bold and caps)

Its disjunctive so the R might have had the definition memorized and in mind.