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From Law 25:
A. Unintended call
1. Until his partner makes a call, a player may substitute his intended call for an unintended call but only if he does so, or attempts to do so, without pause for thought. The second (intended) call stands and is subject to the appropriate law.
2. No substitution of call may be made when his partner has made a subsequent call.
3. If the auction ends before it reaches the player’s partner, no substitution may occur after the end of the auction period (see Law 22.B.1).
4. If a substitution is allowed, the LHO may withdraw any call he made over the first call. Information from the withdrawn call is authorized only to his side. There is no further rectification.
Duplicate Decisions goes on to explain:
Until his partner makes a subsequent call, the Director should permit a player to change an inadvertent call without penalty, provided he changes or attempts to change the call without pause for thought. Inadvertency means a slip of the tongue (or with bidding boxes a slip of the fingers — a mechanical error) has occurred, not a change of mind. The player does not have to make the change of call; any indication that the first call was not his intended call is sufficient. Also, this indication of inadvertency must be made without pause for thought. (The elapse time here should not be your prime determinant. It is important to note that the thought being described is about what to call, not about what to have for dinner.)
Note: Many players have learned to say “mechanical error” whenever the director is called. Directors should be wary of accepting this self-serving statement without acting with due diligence. We do not accept “mechanical error” when the first call is Pass and the second call is 3♠. We should also look askance at hands that hold two long suits where it is possible the bidder was thinking about his second bid while making his first. The onus here is on the player to prove it is indeed a mechanical error, not an error brought about through distractions, fatigue, or mental carelessness.