Going Wrong in Each Direction

A K J 10 8 7
Q 7 2
7 3
K J
9 Q 5
A 8 6 5 3 10 9 4
J 10 8 6 4 9
Q 6 10 9 8 7 5 4 3
6 4 3 2
K J
A K Q 5 2
A 2 Contract: 6 no trump

Something went wrong both ways on this hand. A couple of declarers made only 11 tricks while a couple made 13. For the former, it was disastrous. They might as well have been in grand slam, going down. Their score was 1.56%. The defenders who allowed an extra trick fared a little better, with a 29% score. Anyway, to the declarers first.
I don't know what they were thinking. Take a round of spades to see whether you have a twice-guarded queen on your right first. If you've got it, everybody's got it and you're going down in slam, but you don't want to make it worse. Then . . Well, a simple count shows that you've got six spade winners, two club, 3 diamond for 11, and knocking out the ace of hearts will give you a 12th. A line that shouldn't tax a novice.
Oh, sure. If diamonds split, you've got 13 tricks! Yeah, right. You'd do well to go with the odds, which do not favor a 3-3 split. And even if they did, those in little slam would do well to consider the safe play for their contract rather than to jeopardize it for an overtrick. Anyway, the first declarer took the opening Q of clubs lead with the king, cashed two spades, then ran three top diamonds! ! ! ! You've got to develop, q.v., that 12th trick and in so doing, you don't want to uncover diamond tricks for the defense! A simple count should have guided this declarer. Perhaps on a 4-2 diamond split, there could be some justification in that declarer wouldn't know whether he had a 3-3 or 4-2 split after two leads. And even if it's 4-2, the person with the ace of hearts won't necessarily be the one with the long diamonds. But this declarer had warning that the suit wasn't splitting 3-3 before cashing his last top diamond. It just makes no sense.
The other declarer took the opening diamond lead with the ace and ran six spades. Since he's not uncovering any winners for the defense, that alone didn't do him in. The problem was that he sluffed -- now hang on -- his hearts, the K J, instead of low diamonds. Now when he came to his diamonds, he found they didn't split, which first, he certainly shouldn't have counted on, and secondly, as mentioned, could have tested them for a 5-1 split without risk. So now he led to dummy's king of clubs and led away from the queen of hearts, the ace of that suit and the 10 of diamonds taking the last two tricks.
This declarer calls himself an "ex-expert", whatever that means, but it appears twice on his page, so it wasn't a typing oversight.
Now for the luckier declarers. They each got a low heart opening lead. Now, let's see. That's six spades (we'll test the suit immediately), three diamonds, two clubs and a heart for twelve tricks! And what do you do when you can win every trick but one? Think squeeze!, q.v. Declarer should now cash his clubs (after one spade lead), then run spades sluffing a heart and a diamond. He then has queen low of hearts plus two low diamonds in dummy, opposite A K Q 5 of diamonds. If West has four diamonds to guard that suit, he can't have the ace of hearts. It's not a complicated squeeze by any means.
So a tad ironically, the A of hearts lying with the long diamonds, which proved the undoing of one declarer (as far as a 12th trick was concerned), is what makes this squeeze and a 13th trick possible for the other declarer.
Did these two declarers find the squeeze? Well, not exactly. One wasn't tested. On the run of the spades, he got two diamond discards from his LHO. At that point, he still had communication in clubs, and so could test diamonds with impunity. When both opps followed to one lead, diamonds must run. The other got a faulty discard at a point of no return. He only got one diamond discard from West. When he went to diamonds after the run of the spades and cashed the top three, he no longer had the communication for a squeeze. At trick twelve, declarer cashed the second club in the closed hand and West had to decide whether to save the ace of hearts or the J of diamonds, and guessed wrong. Actually, it shouldn't have been a guess. When his partner didn't follow to the second round of diamonds, declarer is marked for five cards in the suit. But that evidently wasn't noticed.