Firstly, how might wrong information by given?
There are several ways; some typical examples are given below:
- when asked a question by your opponents, your partner gives a
wrong explanation
of your bid;
- your partner
alerts
a natural bid (eg he thinks you are playing transfers, but you are not);
- your partner
does not alert
(or announce) an artificial bid (eg transfer or Precision club).
So, what should you do?
If you are declarer or dummy you should explain the mistake when bidding ends. If the defender has made his opening lead (face down we hope), then he can change it. Explaining at this stage means that the defenders cannot later claim that their defence was harmed. (Of course, they might still claim to have been harmed during the auction, but at least you minimise the damage.) You must not explain the mistake before the bidding ends, as this would give information to your partner too.
If you are a defender, you should explain when play ends. You must not explain before play ends, as this would give information to partner.
What is "wrong information"?
In a perfect world:
- you and partner each have a system card
- the two system cards are identical
- the system cards cover all eventualities!
But this isn't a perfect world.
So when is an explanation (or alert or non-alert or announcement) giving "wrong" information? If the situation is covered on your system card, and partner's explanation differs from your system, then this is clearly "mis-information". But suppose your cards are different! (eg yours says, transfers; his does not). Or suppose the situation is not covered by your cards. Then the question of what constitutes "wrong" information is more complex, and would need a decision by the director.
A common example is where 1NT is doubled. Do you play exit transfers or natural? Perhaps you are not sure whether you partner's bid is natural or transfer. For instance, with some partners you play transfers, with others natural, and you cannot quite remember what you play with this partner. No doubt you feel that you should say something to your opponents, but are not sure what to say. Actually, in this situation, you should not alert or announce, and you should say nothing. Moreover, if you are asked the meaning of the bid, you should just say "no agreement", or "I don't know", or something equivalent. You should not add "I assume it is ...", as, once again, this gives information to partner. Of course, if you "complete the transfer", your partner might guess how you have interpreted his bid, but there is no sin in that. If you are sure what partner's bid (should) mean, then, if it is a transfer, you should alert (but not announce).
This example illustrates a general principle which should apply in all bidding situations. We are trying to ensure that our opponents have the same understanding of the bidding as we do, but without giving addition information to partner.
If as declarer you think that opponents have been given wrong information, then, at the start of play, it is only courteous and friendly to tell them. And your partner (dummy) can give his understanding too. All such information might help the defence interpret your bidding and thus your hand. And it is only right and proper that you should offer the defence this chance.
None of us want to get a good result by an opponent's mistake due to misunderstanding OUR bidding. Opponent's mistakes due to misunderstanding their own bidding are fair game of course.
Lastly, what if should you do if you have made a mistaken bid. For example, you thought you were playing transfers, but your system cards both say otherwise. In this case, technically you have "psyched", and you are not obliged to tell defence. However, you might still feel it is more friendly to tell them anyway!!
And, while on the subject of psyches, it is worth noting that if you play 1NT as 12-14, it is not illegal to open with points outside this range, eg 11 or 15. However, partner must bid as if you had 12-14.