In the 2007 game against Florida, OSU went for it on fourth-and-one from their own 29 yard line in the second quarter. That's about the "least Tressel" moment possible - handing the opponent at least three guaranteed points (possibly more) by not punting the ball away. And the game was only 24-14 at that point, with plenty of time left to even things up.
PJ Fleck described the moment on the sideline:
"Jim Tressel is on the headset in the second quarter going, 'If we don't get this fourth down boys, the game is over,' " Western Michigan coach P.J. Fleck, a grad assistant on that Ohio State team, said in his press conference this week. "And I've never heard Jim Tressel ever panic or ever say anything (like that). You knew you were in trouble." 
And he was basically right, I think. UF's defense was giving our O a pretty significant FU in that game. Just wasn't going to go unless we could get yards.
Another least-Tressel Tressel moment (though I have to give Ramzy the nod here) was the fake against the scumbags in the 2006 tilt leading up to that game: everyone knew it was going to be Beanie for the yard on 2d and one, but the fake faked out the camera man, too.