Player A makes an otherwise legal service that contacts the net on its way over. Player B (the receiver) hits the ball before it bounces on his side of the table.
It is difficult for the umpire to tell if the ball would have bounced on the receiver's side of the table had Player B not volleyed the ball.
The server believes that the receiver has obstructed his "let" service. The server believes he should be awarded a point or a let. The receiver believes the server has failed to make a legal service, and that the ball would have been "long" if not otherwise volleyed.
What is the umpires ruling?
This is according to the following laws of table tennis:
Law 2.5.8 A player obstructs the ball if he, or anything he wears or carries, touches it in play when it is above or travelling towards the playing surface, not having touched his court since last being struck by his opponent.
and
Law 2.9 A LET
Law 2.9.1 The rally shall be a let
Law 2.9.1.1 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 obstructed by the receiver or his partner;
In the case above, Player B has obstructed the ball since he touched the ball when it was above the playing surface. It does not matter whether the ball was going to bounce on his side of the table or not. Since he has obstructed the ball, and the serve was otherwise good except for touching the net, the umpire should call a let, according to laws 2.9.1 and 2.9.1.1 .
