That was the situation last night, in a teams match at the Buchanan Bridge Club. Here's the full deal and auction:
I sat East and despite the vulnerability went for a risky third in hand weak 2♥. South doubled and I got a bit worried when Phil raised me to 4♥. As he said afterwards, he was stretching, and I'd already stretched. South now bid 5♦, passed out, and hit a fine dummy. On the first trick Phil lead the ♥K, which of course declarer won with his Ace.
At trick two declarer called for a Heart, but dummy played a small Club. I was East and also heard "Club" so played a Club, and declarer didn't notice the error so discarded a Club. Phil also thought declarer had asked for a Club, and saw three Clubs in front of him, so of course followed suit and won the trick with his ♣A. At this point declarer noticed the error and the director was called.
The director (who was, incidentally, our team-mate!) ruled that there was no way of telling who had said what but since declarer had played a card himself without correcting the mistake the trick stood.
Fortunately, it didn't matter as declarer still had twelve tricks, and can never get more than twelve. At the other table our team-mates collected 1100 for 5♥x-4.
The event was the Buchanan Congress Teams, for which Phil and I had partnered Ricky Finlayson and Horst Kopleck. They produced a couple of great boards, including 1♦xx-2, and we made a lot of games and bid three slams. In fact the only two game swings out I can blame myself for. Firstly I ducked an Ace against a slam ("Classic Danny" Anna said with delight when I told her), and on the other one we failed to find 4♥:
I transferred to Hearts, went as far as 3♥ but couldn't quite make it to 4♥. We beat 3♠ by a trick but lost 11 IMPs. About half the tables in the Men's and Women's congresses got to game.
Overall we finished on 110 VPs out of 140 to top the standings.