[swanway]

Could you please give a ruling on the following problem.

East plays a low club. South who is declarer plays low. West plays the JC. In dummy there is the Axx of clubs. Declarer says play low. He then realises his mistake and says 'No play the AC'. Do you allow the change or is it too late? Law 45C4(b)says 'Until his partner has played a card a player may change an unintended designation if he does so without pause for thought'. Under normal circumstances a Director does not witness the actual incident and he has to rely upon statements from the different pairs. These statements can often vary. How can you make a judgement call when the facts are not clear?

[blackshoe]

You just have to do the best you can, and hope that players won't deliberately lie to you (IME, they won't) or at least that you'll know if they do.

If "he then realizes his mistake" is an accurate description of what happened, then this is not a case of unintended designation - at the moment he called for the low card, that's what he intended to do. So I'd rule the low card is played.

When the facts are not agreed, Law 85 applies:

Quote

When the director is called upon to rule on a point of law or regulation in which the facts are not agreed upon, he proceeds as follows:
A. director’s Assessment
1. In determining the facts, the director shall base his view on the balance of probabilities, which is to say in accordance with the weight of the evidence he is able to collect.
2. If the director is then satisfied that he has ascertained the facts, he rules as in Law 84.
B. Facts not determined
If the director is unable to determine the facts to his satisfaction, he makes a ruling that will permit play to continue.

Here's Law 84:

Quote

When the director is called to rule on a point of law or regulation, and the facts are agreed, he rules as follows:
A. No Rectification
If no rectification is prescribed by law, and there is no occasion for him to exercise his discretionary powers, he directs the players to proceed with the auction or play.
B. Law Provides Rectification
If the case is clearly covered by a law that prescribes the rectification for the irregularity, he determines that rectification and ensures that it is implemented.
C. Player’s Option
If a law gives a player a choice of rectification, the director explains the options and sees that the choice is made and implemented.
D. Director’s Option
The director rules any doubtful point in favor of the non-offending side. He seeks to restore equity. if in his judgment it is probable that a non-offending side has been damaged by an irregularity for which these Laws provide no rectification, he adjusts the score (see Law 12).

[bluejak]

As a general rule, when a player changes or attempts to change an unintended designation or call we tend to be generous in considerations of time. So unless there is clearly a pause for thought after the realisation of the mistake we do not rule it out of time.

But as Ed says, the circumstances you quote sound like a change of mind not an unintended designation so no change would be allowed.