[The Other Harry]

The center may adjust the ball ONCE when he first touches it. Any other movement is considered "illegal procedure".

That still begs the question of what is considered a snap.

[Echo Talon]

after the ball is adjusted, the line must be still 1 full second including the football... soon as the ball moves its a snap even if the center doesn't snap it, if he just moves it to the side its still a live ball. Usually when this happens the refs will blow the whistle and call illegal procedure/false start on the center. IF the ref doesnt call false start the defenseive lineman can knock the ball out making it a fumble and live for anyone.

[Echo Talon]

But usually what happens when the center does a false start, is the defensive lineman fire off the line and none of the offensive players move, causing the ref to blow his whistle because the defensive man is unabated to the quarterback. That is a rule to save the quarterback from getting killed. The the refs will have a conference to determine if it was a a false start on the offensive or encroachment by the defense.