Law 25A, Law 27A, Law 72C - or something else
The hand don't really matter but it is board 24
https://www.bridgewebs.com/cgi-bin/bwop/bw.cgi?pid=display_rank&event=20220517_1&club=durham
North opens 1 Spade and South bids 4NT.
North now bids 4 Spades and East passes.
South now notes that the 4 Spade bid is insufficient and calls the director.
The director notes that Law 25A states
"If a player discovers he has made the bid that was not intended he MAY... substitute the call he intended for the unintended call"...
North now decides NOT to substitute the call they intended. This is now an insufficient bid that has been accepted - and the auction progresses. (South rebid 4NT and North passed).
I didn't like having to make this ruling (allow play to continue in 4NT), but couldn't think of a way of changing it (if indeed I should) - other than 72C.
(4NT made 11 for a good score)
Comments
You do not say if N claimed his call was unintended (see later).
Do you believe N deliberately made an insufficient bid? If so to what end? If he wants to end up in 4NT he can just pass.
If they are trying some sort of clever Blackwood that allows them to go back to 4NT then as you say Law 72C applies (and it's an undisclosed illegal agreement).
If not then it looks OK - odd, but OK. They've just been lucky.
My only caveat is that if S knew N's call was unintended, he has UI. Quite what sort of hand that would suggest over and above the 4S (which S is entitled to know) is beyond me.
p.s. Having looked at the hands, I presume the players in question were actually EW.
Yes - sorry: they were sitting in the NS seats but the round was arrow-switched.
Hard to argue with this ruling, on the face of it North has made an insufficient bid and it's been accepted. Under those circumstances South is permitted to bid 4NT.
As Jeremy says, there might be some unauthorised information there. But with the 4S being insufficient in any event I can't work out what it might really suggest either. There's a blurred line here, since an insufficient bid is generally unintended, usually assumed to be lapsed concentration rather than a mistaken bid.
It seems to me the presumption I would normally make about the 4S bid (if, for example, partner bid it) is that it is an 'insufficient' 5S showing 2 key cards and the Q. If it's a mistaken bid, and I know that because of UI, then maybe that's not so clear and it suggests not bidding the slam. But this feels speculative.
The IB was accepted and that's the end of the matter.
The bid may not have been "unintended" in the sense of a mechanical error – it's quite possible that N intended to bid 4S but confusedly thought that it was sufficient (e.g. maybe N mistakenly thought that S had bid 3NT rather than 4NT).
If East bids over 4S rather than calling the Director, then the 4S bid has been accepted and there's no further rectification involved. This doesn't seem like a particularly inequitable outcome to me.