Pool Rules on a Table Scratch
The Billiard Congress of America defines a scratch -- a shot on which the cue ball is pocketed -- as a foul. Even if you successfully pocket an object ball, you scratch if the cue ball drops into any pocket and you lose your turn. The rules also note that if a pocket is full of balls and the cue ball touches one of the pocketed balls, it is a scratch. As one of the more common ways to commit a foul, a scratch can happen on a break or during game play.
Break Scratch Rules
If you scratch on the break, any ball that drops into a pocket stay pocketed. Your opponent gets the cue ball in hand, places it behind the head string and takes the next shot. The head string is typically indicated at the end of the table, opposite to where the rack is set, by the second small dot or diamond on top of the side rails. When playing eight ball, if you scratch on the break and pocket the eight ball, your opponent has two options -- re-break or shoot after re-spotting the eight ball behind the head string.
If you scratch while attempting to pocket the eight ball and the eight ball was not pocketed, you don't lose and the game continues. Your opponent gets the cue ball in hand, places it anywhere on the table and shoots.