Both Ways

A 7 5 2
10 9 7
J 3
A K 5 4
9 6 4 Q
K J 6 3 Q 8 5
10 2 K Q 9 5
8 6 3 2 Q J 10 9 7
K J 10 8 3
A 4 2
A 8 7 6 4 Contract: 6 spades
------ Opening lead: 3 of hearts

Two people failed to recognize the significance of their long diamonds here, one a defender, the other the declarer. And the irony is further highlighted by the fact that the declarer who got the most devastating opening lead (the heart) made the contract, while the one who got a benign lead (the K of diamonds, played by North above) went down -- two. Well, lemme start with the first declarer:
On opening lead, East goes up with the Q of hearts and declarer wins with the A. Declarer now cashed the K of spades, and surmising correctly that the Q of spades was a singleton, now finessed the 7, on which East sluffed a diamond, and declarer continued with two top clubs, sluffing hearts. A diamond! Why on earth a diamond? She has five clubs and any notion that this club suit would play out to the 5th round is farcical. Indeed, since dummy is showing four clubs, nobody else could have 5 clubs. Further, this defender discarded that valuable card at her first opportunity, a phenomenon I've referred to any number of times. It's just baffling that anyone would wanna sluff from the four-card diamond suit instead of the 5-card club. Why not discard a totally useless 5th club and await developments? The very next lead would be a club, declarer showing out, whereupon East could see that her partner would be able to protect the third round of clubs. For that matter, East's low hearts certainly don't figure to have any effect on the hand once the Q was played. But out went the diamond.
In any event, that was the end of the defense. Declarer can lose a diamond, cash the ace and now need ruff only once to establish his diamond suit and claim. And there's nothing the defense can do about it (then).
Could retaining the diamonds have defeated the contract? It certainly looks that way to me. Declarer can keep West out of the lead, precluding another trump lead by leading the jack and ducking a cover by East. But East now leads a club, and declarer is looking at two trump in each hand. We see that he could cash the ace of diamonds, ruff a diamond, ruff a heart, ruff a diamond, at which point he'd have the high trump and established diamonds in the closed hand with one major problem: He has no outside entry to that holding and West still holds a trump.
But a declarer who is relieved of the need to ruff more than one diamond can't easily go wrong on a 3-1 split. Making on the nose.
The other declarer got the K of diamonds opening lead, playing from the other side of the table, of course. He might well have ducked it to give himself access to the second round and thus third round of diamonds whichever hand he regains the lead in, but that wasn't what did him in. Declarer now drew trump in three rounds, and that's what did him in.
I said less than two weeks ago that I could build a separate category on 9-card trump suits, 5-4, that would be a piece of cake on a 2-2 trump split, but spell the careless declarer's doom on the 3-1. Oh, they are legion. The hand would also be a piece of cake (on a diamond lead) if that suit split 3-3. But it wasn't to be. Both split on the likeliest distribution, and with the third round of trump, by trick 4, declarer had queered his chances. (The defender above queered the defense on trick 3.) For one thing is certain: you can't ruff two diamonds with one trump.
This declarer now lost a diamond to East, and that worthy switched to a heart, won by declarer and a diamond was ruffed with the 7 (the closed hand to him). Now he cashed two top clubs, sluffing hearts, leading to this end position (rotated to make declarer South, with the lead in that hand):

10 8
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8 7
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8 K 6
9 ------
Q J 8 6
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10 9
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6 5

Well, he has to go down; there's no question about that, and I already said he'd queered the contract at trick 4. But he doesn't have to go down two. Clearly with two trump and two diamonds, he can ruff anything, knock out the top diamond and claim a good dummy for down one. It's gotta beat down two, no? But declarer didn't seem to recognize that he had a winner coming in diamonds. What he actually did was to lead the 9 of hearts sluffing a diamond, taking that phrase "sluffing a loser on a loser" in the wrong direction. One of those two diamonds was a winner, in essence, though the other was a loser. What he actually sluffed was the winner. He still had the loser.