I don't direct much these days, but yesterday I was called because the playing director was busy. Matchpoints, my table was the only one to reach the par contract of 4♠EX-1, which gave us (N/S) around 40%.
2♦ was not alerted, upon inquiery from North it was explained as "Natural". Both pairs are regular partners, N/S is a relatively weak married couple, E/W are very good clubplayers. 3♦ made the normal 10 tricks and I was called after the play. I tried to ask North what he would do if 2♦ was explained as both majors and it seemed like he really didn't know. After thinking for a while he said he would bid 2NT which his partner would raise (he had seen all the cards and knew 3NT was making). E/W didn't mention this, but I strongly suspect that N/S exchanged some UI leading to Souths final pass of 3♦. I also didn't quite understand why South bid 2♥, but West was very clear that he would bid 2♥ if she had passed, and to me pass doesn't seem to be a logical alternative (partner got maximum 4 diamonds).
When I consulted with the playing director after the round I was surprised to hear that he believed this should be ruled as mistaken explanation, but with "No agreement" as the correct explanation. He said this was the general advice given in a European TD-seminar recently. I would have ruled "both majors" as correct explanation, both because my impression was that it was the agreement and because otherwise the nonoffenders are placed in an impossible situation. All 4 reasonable calls by North, Pass, X, 2NT (I don't think they play Lebensohl) and 3♦ has a meaning depending partly or very much on the meaning of the "undiscussed" 2♦. This will often lead to the sort of UI-problems for the originally non-offending side that I expect actually happened.
The TD suggested an adjustment to 2♦W-5 which I accepted (this gave N/S about 75%, 3NTS= would give them 90%). How would you rule and what do you think should generally be ruled as "correct explanation" in cases like this (including when you really belive "No agreement" is correct)?
Edit: When I asked West said he believed 2 Diamonds showed the majors, East said he didn't remember that they had talked about it. They had, like most other pairs in this clublevel tournament, no CC or other documentation.