Surprise!

A Q J 7 6
K
J 9 4 3
J 5 4
5 2 10
J 9 8 Q 5 4 3 2
10 7 2 K 8 5
A K Q 10 3 9 8 6 2
K 9 8 4 3
A 10 7 6
A Q 6 Contract: 4 spades or maybe 6
7 Opening lead: various

Here is an interesting hand, played from both sides of the table, as you might have guessed. Which would you choose for a positive score? The defense or declarer? I confess I would have been wrong. The hand offers a chance for heads-up defense inhibiting 12 tricks, well, for the most part, and a stroke of brilliance on one declarer's part, a squeeze that can't be beat, which was the surprise.
Nine declarers are down in 6 spades, one made 6 with an overtrick, (nobody making 6, bidding 6!). Of those in game, one made 3 overtricks, and 6 made 2 overtricks. My first thought was that some stingy anal-rententives had held tightly to their king of diamonds rather than cover the jack. Well, that wasn't totally wrongheaded, to be sure, but there were other ways by which the defense handed over the diamond suit unnecessarily.
The declarer who made slam with an overtrick, playing from the North hand, got a low diamond lead, and the rest is history. If you get a gift in diamonds for a 12th trick, then obviously it's only a matter of whether the defense cashed their club trick or not for a 13th trick since diamonds split 3-3.
How about 3 overtricks in 4 spades? Curiously, it was the exact opposite fault, a squandering of that valuable king of diamonds. Played as above, West led the diamond 2, and now declarer and East traded mistakes. If declarer had finessed the 9 of diamonds, East's goose would have been cooked. Since the 10 is with West, the finesse of the 9 has the effect of giving dummy the J 10 of diamonds, and there would be nothing the defense could do to prevent declarer's picking up the whole suit. But declarer didn't play the 9, and now if East will just play the 8, saving that king to cover the jack, that will inhibit that easy pickup of the diamond suit which was a piece of cake when East went up. In Third Hand High, I have pointed out that one reason for not going up is to cover an honor in dummy. This will often have the effect of gaining a trick, sometimes of inhibiting an entry, sometimes both. Here obviously there will be no shortage of entries to dummy, but the play of the 8 probably (though not necessarily, because of the squeeze mentioned) would have inhibited a pickup of the diamond suit.
That leaves six people in game making 12 tricks. Lemme see, starting with those playing the hand as above: Okay, the first was as suspected. Diamond jack uncovered. You've got to cover on these balanced suits, if only to retain the respect of your partner when it makes no difference. The second case is even worse! This was a failure to cover on the second round of diamonds. Declarer had already finessed the queen! Does East possibly think declarer now won't push the jack through? It makes no sense. Hey, fella. Defense is a cooperative venture, no?
Now the next three were played from the North hand above: Oh, an entirely different error. Heart opening lead, the diamond hook was taken at trick 5, a club was sluffed on the ace of hearts at trick 6, a heart ruffed, then a diamond to the ace, and a diamond continuation, and now a "safe" queen of hearts which declarer must ruff. The problem was, of course, that the defense never got around to cashing the club trick! This was covered in Only Suit Left. You can't get any more heart tricks. Declarer has pul-lenty of trump. You can't get any diamond tricks. There's only one left, and if declarer doesn't have it, he can ruff it. The only suit where you can get a trick at that point is clubs! It doesn't matter whether you lead from a king, from a queen, you might as well hit clubs, for it's your only chance for another trick.
Oh, still another different error, this time by the one holding the 10 of diamonds. The diamond hook was taken at trick 5, and now at trick 10, this defender sluffed the 7 of diamonds on a spade lead. He did it to save both the Q and 10 of clubs. Declarer is out of clubs in both hands at that point, so that defender couldn't possibly win a trick in clubs -- unless he got the lead in another suit. No, this was clearly the 10-holder's fault. When the jack of diamonds was led, it was covered with the king, taken by the ace, and the 10 fell. Now the 9 won the next trick and the J and K of spades went on the last. Plenty of trump. This is, I suppose another illustration of Only Suit Left. You can't win another trick (at that point) except in diamonds! So you'd better save your diamonds.
And the last one was a plain vanilla refusal to cover the jack of diamonds. Such a simple cover. C'mon. You don' think your partner can have the 10? Which brings me to the pièce de résistance, a perfectly played hand, a squeeze I admit I didn't foresee until I looked to see why a defender had discarded a low diamond "when he didn't have to". It turned out he had to, or give up a trick in hearts. Here's how the hand was played:
Club opening lead (played from the North hand above), continuation in clubs, ruffed. Two spade leads, unblock the king of hearts, diamond to the queen, ace of hearts, sluffing a diamond, ruff a heart (West now has no more hearts), ruff a club, spade to the queen, cash the last spade, the 7 and . . .
Declarer holds the J 9 of diamonds. The East player above (looking at the South hand as dummy) holds the K 8 of diamonds and the queen of hearts, while dummy is showing the 10 of hearts and A 6 in diamonds. And he's gotta discard something, no? Well done. Very well done.