3D was interpreted by North as natural, intended as a try for no-trumps by South. East asked before leading whether "everything was natural" and was told by North that it was. South did not volunteer any correction.
The table result was 3NT+1 by North on a heart lead. The TD ruled that West would still pass an alerted 3♦ some of the time. East would lead a diamond and 3NT would go three down. If West doubles 3♦, North will bid hearts and reach game which may make and may go off.
The TD assigned a weighted score of 50% 3NT-3 by N, 25% of 4H-1 by N and 25% of 4H= by North.
The AC considered the ruling to be slightly generous to EW and changed the ruling to
20% 3NT-1, 20% 3NT-3, 20% 4H=, 20% 4H-1, 20% 4C-1. In particular the AC considered that the TD should have considered a significant percentage of a non-diamond lead against 3NT, in the actual auction where 3D was not doubled.
It was agreed that 3D was alertable, and that South should have volunteered a correction before the opening lead that she intended 3D as artificial, seeking help for no-trumps. South was asked why she did not do so, and she stated that she did not think that she needed to with a bid of the opponent's suit.
NS were content with 100% of 3NT-3 (!). EW appealed as they regarded the weighted score from the TD to be inadequate as East stated that if South had said that 3D was not natural she would have led a diamond. The AC considered how much more likely East was to lead a diamond after the correct explanation, and decided that 50% of a diamond lead was generous to the non-offenders. A separate poll since suggests that a diamond lead would be the normal choice by a big margin, but the key issue is how much more likely one would be to lead a diamond with correct information.
Both NS and EW were county level players (perhaps US life master strength). Your views?