Shouldn't Make -- Unless . .

10 3
A 4 3 2
J 4 3 2
K 10 2
J 9 6 Q 7 5 4 2
10 9 J 8
Q 8 5 K 9 6
Q 9 7 4 3 8 6 5
A K 8
K Q 7 6 5
A 10 7 Vul: None
A J Contract: 6 hearts

This contract should not make. Well, okay, it should not without a successful club hook, which I didn't see anyone taking. The key suit here is the diamond (though clubs became the key suit in a couple of instances which will be given shortly). When you're looking at A J 10 not all in the same hand, and you have neither the 9 nor the 8, then you don't have a genuine double finessing position. With the 9, it won't matter how the honors are placed (i.e, if they're not both back of the A), and with the 8, you have some hope of smothering the 9 for what then becomes substantially a double finessing position.
Now, there is a way of escaping with only one loser in the suit -- provided one opponent has a doubleton honor and you guess right about which opponent. I won't go through all the permutations here. Suffice it to say, there is that potential and conversely, when each opponent has at least three cards in the suit, you can't manufacture two winners with honors against good defense.
On the above hand, six people were in slam, three making, three not. All who made the hand got a defensive error, to wit: In case one, declarer ducked the low lead from dummy, playing the 7 and West went up with the queen. I looked at that a second time, ran my eye along West's played cards to see if he'd somehow played the 8 earlier, though that was impossible since there hadn't been a suit led that he couldn't follow to. I can only call it an inexcusable blunder, for declarer now had a simple finesse against the king, which was indeed handed to him when West continued the suit.
In case 2, there was a contrary error. When declarer led a low diamond from dummy, East went up with the 9! Declarer played the 10 into the queen, and now when West continued the suit,, finessing himself out of the value of the 8, declarer ducked in dummy, and East's king was pickled. He chose to play it, and now the jack took the third round. Splitting honors is one thing. But when you have a single honor (or high spot) that declarer may or may not finesse against, you don't want to go up and hand the card to him. Declarer may not be intending to finesse, or, as in this case, your partner may have the cards that would render the finesse unproductive anyway. So you don't want to hand over those valuable cards. Here, the ten would go to the queen, and East on a low diamond would hold the K 9 tenace over the jack.
And in case three, West returned a club, which declarer let ride, and now, having wisely drawn two rounds of trump without using the ace, declarer as able to cash the ace of clubs and go to dummy with the ace of hearts to cash the king of clubs, sluffing a diamond. (Another declarer in 4 hearts got the same club return after playing the 10 of diamonds into the queen. Only he'd cashed one round of trump with the ace, and had cashed his spades, ruffing the third round, and so couldn't make use of it!) A diamond return to his partner's K 9 tenace would -- probably --have scuttled the slam. Still, declarer had retained the potential for 3 club winners himself, and on a "make or down two basis" just might have found a winning club hook, the ace of hearts serving as an entry to make use of the third club winner.
Then I took note of all the people making 12 tricks though only in game. While the slam bidders divided 50-50, here there was a marked disparity of 24 pairs in game making 12 tricks and only 6 making 11. I didn't want to go through all of them, but spot-checked three and found: Well, one got his first round jack of clubs covered. At first I thought it had to be an improper cover, as it certainly would have been had the ace been played first, in which case West would have been able to see that no gain could come from the cover. But given that the ace hadn't been played and these people weren't in slam, making the ace much more difficult to place, I'm not so sure it was a bad cover. I think West gets a pass on that one.
Another declarer got an opening club lead. And a third, surprisingly got the same 9 of diamonds hopping up on a low diamond lead as the defender described above did against a slam, plus West's continuation of the suit, which is necessary to complete the gift. Of course there are no absolute rules in bridge, and one defender's squandering of a valuable 9 on one hand is another defender's clever unblocking in a slightly different situation. Nevertheless, I would advise against trying to be too clever by half. You basically want to guard your honors and high spot cards until you're making some use of them. Don't hand them over.