This was a great trick play that capped Clemson's 21-0 disaster in the first quarter. Sorry for the video quality. GT WR #8 Thomas either comes into the game at the last second or just never steps off the field when the offense runs off (more likely). This seems like it's right on the edges of the rulebook. Someone who knows more about the rules on illegal shifts and substitutions should explain.
Another thing that makes this play so great is that GT just scored on a fake FG punt by Clemson (which was just so so stupid). Clemson ended up tying the game but they would have won if not for those two absolutely brain-dead touchdowns.