Lessons

10 9
Q 4
A 9 3
A Q 10 9 5 2
Q J 7 4 3 K 8 5
9 8 5 3 J 10 2
K 8 6 5 Q 7 4
------ J 8 7 3
A 6 2
A K 7 6
J 10 2 Contract: 3 no
K 6 4Opening lead: low spade, Q of spades

A marked disparity in tricks taken in the same contract, often with the same opening lead, will trigger suspicions that somebody has made a serious even if elementary goof. A disparity of 3 or 4 tricks is not all that uncommon. Of course, if one lonely potzer has butchered a hand, that is to say, if it's sheer wrongheadedness without much of a lesson, I'm not likely to zero in on the hand. But here, where no fewer than 3 people failed to make their 3 no contract and no fewer than twenty-one people made 12 tricks, which also seems unwarranted, I felt it was worth a look.
Opening lead (almost always) was a low spade (a few times the queen). On first glance at the spade suit, I thought declarers had failed to employ the hold-up. But when I took a gander at the clubs, I saw where the problem lay. Yes, three declarers initiated the club suit with the king. So now they can't pick it up without loss of a trick, and not having held up on two rounds of spades, couldn't afford to lose the lead, and so were held to 3 clubs, 3 hearts and two aces for down one.
That shouldn't have been. With 9 cards, missing the jack, you want to protect against a 4-0 split in either hand -- if you can. Which is to say, you here lead to the hand with two honors, since now you can finesse against the jack in either hand. This is not invariable, now, that you lead to the hand with two honors. Let's give East the 10 for the 7. Now the king would be right!
That's because if East has 4 clubs to the J 10, you can't pick up the suit without loss of a trick (and should have held up in spades!). But if West has 4 clubs, you'll know it in time to finesse twice against the J 10, picking up the whole suit. This, indeed, should be part of one's repertoire for even shorter holdings:
A Q 10 5
K 7 6

Here with another twice-guarded king, you'll want to go to dummy with the ace and come back to the king in case your RHO started with a singleton, or maybe with 2 and has discarded one, thinking it was useless. Or with eight cards.:
K Q 10 5 4
A 6 3

You'll want to take the first round in dummy so that you can come back to the ace and see if RHO shows out. If both show in, the suit runs. If LHO shows out, RHO has J 9 over the 10, and there's nothing you can do about the loss of fourth-round control in the suit. With a doubleton honor, of course, you'll (usually) start with the honor, lest you block the suit. But with a twice-guarded honor, there's seldom reason to start with that honor. In the above case it was costly because it wipes out your ability to finesse against the jack, while in the two isolated cases, it was so because you might learn too late of a marked finesse.
Enough of that. Now the question is, how come so many people made 12 tricks? The cards seem to offer only eleven. No double finesse with only one spade stopper. Yet 23 did take that double hook in diamonds. I looked into the phenomenon and found that in the first case, it was indeed a double finesse that did the trick, bringing declarer 12 tricks. Opening lead was a heart, prompting declarer to start a double hook in diamonds, and then the continuation was also a heart, relieving declarer of any danger that would have been inherent in a spade shift, i.e., that the second half of the double hook wouldn't work.
Then I started finding that the reason for a 12th trick (on a spade lead) lay in West's discard of a heart, allowing declarer 4 tricks there. I have a bit of a thing about Four-Card Suits (q.v.), something I didn't give a whole lot of thought to until the advent of OKBridge, where one could examine the whole play of the hand later. Now I think it one of the most common defensive errors, and one that is almostly as commonly foreseeable. Unfortunately, I can't quite say it about this hand, one of the few where I don't think West has any clue as to what to save when clubs are run.
Often it's a matter of the bidding, not too rarely because a four-card suit is showing in dummy, maybe a low lead from partner indicating (not altogether promising) 4 to an honor should prompt a defender into holding the right suit. But here, West has no such clues, and I wouldn't fault him for discarding a heart. After all, the suit doesn't look prepossessing and there are a powerful lot of discards to be made. However, having said that, there are two comments I would make.
One, I think it a poor practice to vacillate between suits. I can't say how many defenders sluffed a diamond on the first club lead and then a heart! So by trick 3, they've taken themselves out of contention for holding the right four-carder in two suits! Nine times out of 10, once you decide on a suit, you're going to do well to stick with it, save one of them anyway. It may be the right one, and maybe your partner will notice and know he must save the suit you're discarding, not one you're presumably saving.
What was a little disheartening was the defender who started out so right and then collapsed, as it were. On the first three club leads, he sluffed diamonds, blanking his king. Then he followed to a lead to the queen of hearts and . . . the hand should almost have been an open book to him then. If his partner didn't capture the queen of hearts, then you know declarer has 11 winners, in the form of 3 hearts, 6 clubs and two aces. Opening lead was a low spade to his partner's king and ace. Now, at this point, if his partner doesn't have the queen of diamonds, declarer has his 12th trick. So this defender who'd started out so bravely blanking his king of diamonds would have done well to continue with discarding it on the next club lead. It's going on the A anyway, and if his partner has the guarded Q, the K won't be necessary.
Now the rest of his discards might be just a little easier. West can afford 3 cards with ease: the king of diamonds and two spades, he can't win more than two spade tricks anyway. The crunch will come on the lead of the ace of diamonds. Or will declarer resolve the situation for you by not leading the ace of diamonds, but instead, saving it for the end of the hand. That's not at all rare. He can't read your mind. On that scenario, declarer sluffs two spades and one diamond on the 3 long clubs, comes to his hand with the A K of hearts, sees that the 4th doesn't jell, and you've done your work. Now he goes back to dummy with the ace of diamonds -- dropping your partner's queen! Well, that happens, too. You do everything right to guard against the 3rd overtrick and your partner gets careless. But if your partner isn't so foolish as to blank his queen when he sees you discarding diamonds, you will have saved a trick.
If declarer is a little more savvy and after running clubs, discarding 2 spades and a diamond, cashes the ace of diamonds, then I would grant that West doesn't have a clear-cut solution. Did declarer start with 4 spades and 3 hearts or vice-versa? And who would blame him if he guesses wrong?

Footnote: There were 8 who made only 11 tricks. From wondering why so many made 12 tricks, I now got to wondering why so many made only 11! I checked only 3 tables. On the first, declarer held up on 2 rounds of spades, and that's one way to win only 11 tricks. On the next, no, it wasn't a savvy defender. This defender discarded his hearts so aggressively that he couldn't even follow to the third round of hearts. It was rather an unobservant declarer that held himself to 11 tricks.
Then on the 3rd and last run-through I was intending to look at, there was a very sharp defender who did save his hearts, inhibiting a 4th round heart winner for declarer. At trick 11, declarer went to the ace of diamonds, and the defense took the last two tricks with the queen of diamonds and a spade to the queen. Well done. Very well done.