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Maybe this one? 2:11 in the video.
This would have been 2013 so he wouldn't have been backup. Certainly walked off cool and calm though.
I just went back and looked through the 2010-2013 matchups. The games Tommy would have come in as backup in his career would be:
2010 Michigan - Came in after Crist took the head injury, but Nate Montana took most of the reps while Crist was out. Neither QB was prepared, something Kelly said he would rectify in the future. No TDs by either player.
2010 Tulsa - Came in after Crist went down with the knee injury, we didn't hold them off though. Rees did score some TDs.
2011 South Florida - Forgot this one, came in for ineffective Crist to try and start a comeback. We fell just short.
2012 Purdue - Rees came in for the final drive to get us a FG and win in regulation
2012 Michigan - Rees came in for an ineffective Golson, but the only TD he scored was with his feet in a weird twist. Michigan never saw it coming.
2012 Miami - Tommy came in for a few plays, but only because we were dominating the Canes I think. No TDs here.
2012 Stanford - Tommy came in for Golson to finish the game and win in Overtime after a Golson concussion. TD pass to TJ Jones to put us up.
2012 Oklahoma - Tommy went 1/1 coming in for Golson briefly, but no TD pass.
2012 Pitt - Tommy came in for an ineffective Golson, but also struggled. This was the flu game, horrible team performance for 3+ quarters.
2012 BC - Tommy went 1/1 again coming in for Golson briefly, but no TD pass.
2012 Wake Forest - Tommy came in late after we were up big on the Deacons, no TD pass.
Tommy did throw a TD pass against BYU in 2012 as well, but he started the game after Golson was suffering the effects of the concussion from Stanford the week before. Tommy started all of 2013, most of 2011, and the end of 2010, so any of those games would have been as starter.
[EDIT] Forgot to add South Florida.