Partner led the 4 of spades, and dummy came down with: ♠KQ95 ♥8 ♦982 ♣AK864
Declarer called for the Queen. If you play the Ace, declarer plays the 7 of spades. Plan the defense.
Again, these are your hand and dummy's hand:
The contract is 2H. The 4 of spades was led, the Queen from dummy, Ace from your hand and 7 of spades from declarer. What do you do next?
At the table, I returned the Queen of diamonds. It was several plays later that I discovered my error. Much too late, of course.
If partner has the Ace of diamonds, there is still time to capture declarer's king. You have a sure trump entry after all. No, the danger is the club suit. Sure, you have the Queen of clubs, but you need to take out declarer's spade entry immediately. Win the Ace of spades, and immediately lead the Jack!
The full hand was:
p.s. On the actual layout, declarer can unblock his 10 of spades under the Jack, but if he does that, you have the counter play of playing a third spade and getting partner to ruff -- you are unlikely to find this at the table, though. You're more likely to think that your partner started with the 432 of spades. Still, returning the Queen of diamonds only makes it easy for declarer, and gives him time to set up his clubs.