From the latest "Handbook for Match Official":
http://www.ittf.com/wp-content/uploads/2016/09/HMO_15th_edition.pdf
11.2 Net Cord Service
...
11.2.3 If either the umpire or the assistant umpire believes that the ball in service touches the net the rally is a let. Even if there is only a suspicion that the ball touches the net in an otherwise good service it is better to declare a let than to allow play to continue, because there is a risk that one or more players may have the same suspicion and, as a result, will be unable to give full attention to the course of play.
11.2.4 A player who believes that the ball touched the net in an otherwise good service may sometimes raise his or her arm or catch the ball and ask the umpire to declare a let. Unless the umpire is certain that it did not he or she will usually concur, especially if the server agrees with his or her opponent, but he or she should make clear that he or she is under no obligation to do so and that the player should continue playing if the umpire does not declare a let.
These 2 passages are menacing. We need to implement a fail-safe for either side to challenge the calls, rightfully or not, by either ump or the other side.
Take the human component out of the equation, be it integrity, common sense, race, ethnicity etc.