(Based On A Simulation From EBL Workshop – Belfast)
With South the declarer in 1NT, West leads ♥3. Declarer calls for the ‘Jack’ from dummy, East plays ♥Q, and declarer plays ♦K.
North now says ‘No hearts partner – but you opened 1NT’.
South – ‘Sorry I thought the lead was a diamond, and I was actually playing the ♦J from dummy’.
The Director assesses the situation and tells South that he will apply Law 46 ‘Incomplete Or Invalid Designation Of A Card From Dummy’, and moreover 46A basically states that if it is clear that declarer didn’t intend to play the ♦J, he can replace that card with ♥J. East will be allowed to replace his card, and declarer must now withdraw the ♦K and follow suit with a heart. Play would then continue as normal.