[VixTD]

I don't think I'm really in doubt about the correct ruling here, but I just thought I'd check what other people do in this (not uncommon) situation:

2♥..3♣(A)...X...3♦
.P......P.....3♥....P
4♥.....P......P.....P

I don't remember the auction exactly, but the first two calls and the final contract were as shown.
2♥ was weak, 3♣ alerted, asked and explained as "CRO" (a popular Ghestem variant which would show ♠ and ♦ over 1♥, but is not played over weak twos).

Before facing his opening lead, the 3♣ bidder volunteered a correction of his partner's explanation, saying that 3♣ was natural.

At this point the TD is called. I know you're going to look into possible use of UI, MI, UP et al. at the end, but what do you do now?

[RMB1]

VixTD, on 2013-February-21, 08:13, said:

At this point the TD is called. I know you're going to look into possible use of UI, MI, UP et al. at the end, but what do you do now?

I think you should "tell off" overcaller for the incorrect timing of the correction.
In principle you should allow responder to change their final pass - Law 21B1a does not concern itself with how the misexplanation came to light.

[Zelandakh]

I think the "telling off" can wait but what cannot is making sure that both defenders are aware of their UI obligations.

[VixTD]

RMB1, on 2013-February-21, 08:46, said:

I think you should "tell off" overcaller for the incorrect timing of the correction.
In principle you should allow responder to change their final pass - Law 21B1a does not concern itself with how the misexplanation came to light.

That's what I did, but I had a feeling of doubt as I was doing it that the non-offenders shouldn't be getting a chance to change their last call, because they shouldn't have had a correction of the misexplanation, but in retrospect I agree with you that what I did was right.