The ♣9 is led against your 3NT contract, East's Ace winning the trick. A heart is switched at trick 2, West wins the Ace and plays back a heart. What do you discard from dummy?

Solution:
You have 3 top tricks in diamonds, 2 in clubs since the Ace is dislodged, 1 trick in hearts, and require 3 more tricks. If diamonds lie favorably, you have 2 more tricks there and need just 1 trick in spades. However, if East has four or more diamonds to the Jack, you need 3 spade tricks. If you carelessly discard a spade at trick 3, the contract can no longer be made. It is silly to discard a diamond; when diamonds are favorable, as a trick is wasted unnecessarily. The correct discard is a club, retaining your options in both diamonds and spades. After winning East's ♥10 with the King, you can test diamonds by cashing the King and Queen. When West discards, you can change tack and play on spades. Since West has the ♠A, you can develop 3 spade tricks and land the game.

Congratulations if you came up with the right solution. When this deal was played at a pair event, many declarers failed to make the contract when they discarded a spade from dummy rather than a club.

Analysis:
Bridge Baron pointed out that an opening heart lead defeats the contract. West was justified in not leading a heart as you had bid the suit, but notice how effective leading "fourth best from your longest and strongest suit" can be. A clairvoyant ♥A lead defeats the contract two tricks.

Bridge Baron's line of play
Bridge Baron rose to the occasion by discarding a club when West continued with a heart. After winning the ♥K, Baron played a club to the Queen, then tested diamonds by playing the King and Queen. Realizing that the diamond suit did not have any future, Bridge Baron played on spades and made the contract.
Par Contract Analysis:
The par contract on this deal is 3♠ by North-South

Bridge Baron deal No : N3434-91759-27696-11251-61725-80318