A board is considered to be fouled if the Director determines that a card (or more than one) was displaced in the board, or if he determines that the dealer or vulnerability differed between copies of the same board, and the contestants who should have had a score comparison did not play the board in identical form for such reason.
In scoring a fouled board the Director determines as closely as possible which scores were obtained on the board in its correct form and which in the changed form(s). He divides the scores on that basis into groups and rates each group separately as provided in the regulations for the tournament. (In the absence of a relevant regulation the Director selects and announces his method.)
See Law 86B2.