I played in a Charity Teams Event at the Buchanan Bridge Club last week. On one hand declarer won a trick in dummy, then played a card from his hand. The card was already on the table, when dummy pointed out that it was from the wrong hand. I was the defender last to play, and said that I accepted the wrongly played card. Dummy then chimed in to say that declarer can "do what he likes", and could change his card if he wanted to (no one else had played yet). It didn't really matter, to me or anyone else, so declarer put the card away and lead from dummy.
I think I was right though - declarer cannot show a card from the wrong hand then play from the correct hand, as the wrongly played card could be viewed as fishing to see how the defenders react.
Here's the full deal:
Presumably North-South were playing a Strong NT as North opened 1♣. I had the 15 point East hand but settled for overcalling 1♥. The opponents finally settled in 2♠.
I won the Heart lead then switched to a Diamond, which declarer won in dummy. He then lead the ♣J from hand, and I thought "great I'm going to win my ♣K" and so accepted the lead, before we agreed he should play from the right hand. But maybe my eagerness made him suspicious, or he just wanted to continue the suit he'd committed to playing, because he then lead the ♣A from dummy without finessing.
After that we defended well. I recognised the possibility that partner might have the missing ♠Q and ruffed a trick high, and we went on to score Spade tricks separately and reduce declarer to 2♠-1. This gained 4 IMPs when our team-mates played the unlikely contract of 2♣ from South (South was starting an escape from 1NTx and North left it in). Even more surprising is they made it. Why was no one playing with Diamonds as trumps?
The event was the Erskine Charity Handicap Teams. There were eight teams, and before the handicap was applied we scored 68/140 VPs, for 5th place.