Basic Counting

You hold the following hand:
S AQ93
H 985
D 10865
C J8

The opponents have the following auction to 3NT:
1C - 1H;
1NT(1) - 2D(2);
2NT - 3C;
3D - 3NT

(1) 15-17
(2) New minor forcing

What do you lead?

Normally AQ93 is not an attractive holding to lead from, especially at matchpoints when blowing an overtrick is a very big deal, but it sure seems like from the auction the opponents don't have much strength there, so it was the S2 for me. On the first round dummy shows up with a stiff 7 and partner wins the king, declarer following with the 2.

Partner returns the S6 and declarer plays the 4. At this point you can win your 9 and cash out the suit. But this is a bad idea! Remember the auction -- declarer denied having 4 spades when they rebid 2NT over the New Minor Forcing bid. So you know that your partner has 5 or 6 spades, and if you play the 9 now you can't get them in to finish running the suit.

Playing Ace then queen then 9 lets partner overtake, and you have the first 5 tricks against 3NT. A good thing too as the opponents have 11 top tricks outside the spade suit.

Incidentally, at the table, at the sight of dummy the declarer began berating his partner about what a terrible bid 3NT was. While it's always the case on this hand that you can count the suit and know to unblock it, there will be other hands where it's not so obvious. No matter how much you're fuming as declarer, if you can stay calm the opponents are a lot more likely to err.