6D made on the play shown.
The event was played with screens. On the W/S side of the table the 3C bid was described as natural (not alerted and not asked about, 4C was asked about and described as natural). On the N/E side of the screen the 3C bid was asked about and described as Checkback (written response confirming this).
On the play a spade was led. W won the ace and, thinking declarer/N held something like Kxx - AKQJx Axxxx, returned a diamond. As such the slam made rather than going off - as it would have if W had given E a spade ruff.
At the end of the play the difference in explanations came to light. S stated that 'Natural' was the correct explanation for 3C and that describing 3C as Checkback to E was the error - so W had had been given correct information throughout and the result should stand.
The NS pair had a well filled in, detailed, convention card which does not mention Checkback being played over 2NT (but does mention, for example, 2-way Checkback over 1NT). The card is linked below:
NS Detailed Convention Card
The Tournament Director ruled that the result should stand (6D making) - as West had been given correct information throughout the hand.
Do you agree?

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								 Personally I would have ruled in your favour but, unlike the TD, I've not spoken to North or South and perhaps they were sufficiently compelling along with the other evidence. Perhaps they'd also have been compelling at a Camrose weekend too.
 Personally I would have ruled in your favour but, unlike the TD, I've not spoken to North or South and perhaps they were sufficiently compelling along with the other evidence. Perhaps they'd also have been compelling at a Camrose weekend too.