Wake Up
Bidding (unopposed):
E - W
2C(A) 2D(A)
2NT 3H (no Alert)
4H (1)
3H is transfer to Spades.
1) Followed by "Oops" from E
The 2NT is 21-22 balanced. EW have no agreement about breaking a transfer by bidding another suit, either over 1NT or 2NT openings
Is it at all credible that E can have a hand on which he would knowingly bid 4H (natural) over the transfer to 3S?. I would say no.
If not, am I correct that W can "wake up" to his partner's misunderstanding?
Comments
Sure: five good hearts, three little spades and, for whatever reason, an unwillingness to stop at the three-level.
Why is 4H natural?
Because they have no conventional agreement over it, and in similar situations it would be natural. Besides, if it's not natural then it almost certainly agrees spades and so W has no problem bidding spades.
As always, we consider what peers of the player would do in this situation without the UI.
Where a problem might arise is if West has a hand suitable for a slam try only opposite a super-accept. That might get them too high without a good enough fit.
Even without discussion, I'd consider this some kind of superaccept for spades, probably showing good hearts. Whether it's possible for the 2NT rebid to have 5 H might matter.
I suppose with a weakish 5-4 or something pass might be a logical alternative.
Gordon's scenario seems more likely, that the UI prevents a slam try.
To address the original question, E is probably allowed to know the 4H isn't entirely natural. But it is still UI that partner didn't mean to bid it.
As always with UI cases, you need to look at this from a logical alternative point of view and a demonstrable suggestion point of view.
The "oops" suggests that E doesn't hold anything special in hearts (because it implies that E misbid rather than trying to make a, presumably natural, heart bid). This means that W must pick from, among the logical alternatives, the one that is least suggested by E not holding hearts, i.e. the one that's most suggested by E holding hearts.
This might be pass (e.g. if W holds three good hearts), but might also be something like bidding 4NT rather than 4S if it would be natural in the system, and W has decent spades but only a small singleton in hearts (such a holding would suggest playing in spades unless E has hearts well-stopped).