This article is from the Table Tennis (Ping Pong) FAQ, by ttennis@bu.edu with numerous contributions by others.
3.10.1 The rally shall be a let
3.10.1.1 if in service the ball, in passing over or around the net assembly,
touches it, provided the service is otherwise good or the ball is
volleyed or obstructed by the receiver or his partner;
3.10.1.2 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;
3.10.1.3 if 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;
3.10.1.4 if play is interrupted by the umpire or assistant umpire.
3.10.2 Play may be interrupted
3.10.2.1 to correct an error in the order of serving, receiving or ends;
3.10.2.2 to introduce the expedite system;
3.10.2.3 to warn or penalise a player;
3.10.2.4 because the conditions of play are disturbed in a way which could
affect the outcome of the rally.
 
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