They can be a bit ridiculous about the sideline as well. I recall when Vince Young was a rookie, running out of bounds, and running into a defender who was already out of bounds. The defender, a cornerback, just braced himself for the collision, didn't really even step towards Young, but was penalized for hitting him out of bounds, even though the QB initiated the collision.

I recall a Giants-Jets game, where the Jets were down late in the game, and a Giants runner went out of bounds, just as the Jets defender hit him. The defender hit the running back while he was, at most, breaking the plane of the sideline. He had certainly not put a second foot down on it, and might not have even put the first foot on the sideline, although at the moment of collision, he was clearly over the line. But the point is, the ball carrier was protecting a lead in the fourth quarter, and when the defender lunged at him, he was not yet out of bounds, and might not have yet been out upon the point of impact. There was no reason for the defender to assume he was even trying to go out, since, again, he was carrying the ball with a lead in the fourth quarter. But it looked like a bigger deal than it was, because they crashed into some coaches on the sideline, and so the Jets were flagged for hitting a defender out of bounds. That was an issue with Michael Vick in his heyday. They seemed to want defenders to pull up if Vick was close to the sideline, even while announcers were raving about his ability to back cutbacks and dodge. With a QB who is more of a ball carrier than normal, you get burned if you spot him the assumption that he's trying to avoid being hit.