Table Tennis Let Rules:

If in service the ball, in passing over or around the ping pong table net assembly, touches it, provided the serve is otherwise good or is obstructed by the receiver or his partner. If the service is delivered when the receiving player or pair is not ready, provided that neither the receiver nor his partner attempts to strike the ball. If, the failure to make a good service or a good return or otherwise to comply with the Laws is due to a disturbance outside the control of the player. If play is interrupted by the umpire or assistant umpire. To correct an error in the order of serving or receiving or ends. To introduce the expedite system. To warn or penalize a player. Because the conditions of play are disturbed in a way which could affect the outcome of the rally.