From Greg Letts,
Your Guide to Table Tennis / Ping-Pong.
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Courtesy of the ITTF Handbook 2007/2008

2.14 Out of Order of Serving, Receiving or Ends

2.14.1 If a player serves or receives out of turn, play shall be interrupted by the umpire as soon as the error is discovered and shall resume with those players serving and receiving who should be server and receiver respectively at the score that has been reached, according to the sequence established at the beginning of the match and, in doubles, to the order of serving chosen by the pair having the right to serve first in the game during which the error is discovered.

2.14.2 If the players have not changed ends when they should have done so, play shall be interrupted by the umpire as soon as the error is discovered and shall resume with the players at the ends at which they should be at the score that has been reached, according to the sequence established at the beginning of the match.

2.14.3 In any circumstances, all points scored before the discovery of an error shall be reckoned.

Next: The Expedite System