If a pitcher is in the wind-up and the ball falls forward out of his hand towards home plate during the pitch is it a ball? or a balk?
This depends on whether there are runners on base, and whether the ball crosses a foul line.
When there is (are) runner(s) on base: if the pitcher "drops the ball while on the rubber, even if by accident, if the ball does not subsequently cross a foul line", then it is a balk. However, if the ball does reach the batter and then exit the diamond, then it will be a ball (since it will be obviously below the strike zone).
Interestingly enough, if there is nobody on base, then nothing happens: time is called then and then the play is restarted.