Five defenders found four different ways to yield this beatable contract to declarer, a couple of them with downright foolish plays, and I might as well start with one of those, for it illustrates this category.
Opening lead the deuce of hearts, declarer continuing the suit, then played the 3 of diamonds to the Q, holding. He then cashed --- whoa! The Q of diamonds held? I just cannot fathom why anyone wouldn't take that Q with the A. Nor would "mismouse" ring true here, since the card played was the 7, which was two over from the A. I just don't understand. A beginner in his or her first evening of bridge would know to take that trick. Did he expect to capture a better card? Well, if declarer holds the K, West will probably be able to capture it later, though there's little reason for playing such a cagey game, but if his partner holds the K, there isn't a better card for West to pick up! I jus' doan unnerstand.
The second very foolish play: J of clubs opening lead, declarer winning with the A, takes two rounds of trump, one round of spades, low to the Q of diamonds, West winning, now cashing his K of clubs, following that with a low club! ! ! ! declarer winning with the 9! That one's about equally hard to understand. West has the 10 of clubs, the 4 and the deuce and the 9 is showing in dummy. I might even point out that again the card chosen is two cards over from the card he should be playing. Did he expect his partner to ruff? If wouldn't matter if East could ruff. It simply couldn't have hurt to play the 10.
The remaining three defenders coughed up the fulfilling trick, but not in blameworthy ways.
Case 3: Spade opening lead, diamond to the Q and A, another spade, declarer following that with a ruff of dummy's third spade, (a seemingly pointless play, ruffing with the 9 of hearts because he can spare it, but it made the hand). Now declarer led the deuce of clubs to the 8 and East's J. Back came a club, taken by declarer with the A, now a heart to the 10, a diamond back, ruffed with the Ace, diamond 10, covered by the K and ruffed, low club, the 10 from West, East willy-nilly winning with the Q, and yikes! He has only the J 10 of spades left! So he is forced to give declarer a sluff&ruff, declare sluffing the 8 of clubs, ruffing that loser in dummy.
West, of course, "should" go up with the K of clubs with an "alligator coup", i.e., swallowing his partner's Q so as to retain the lead. Should West have figured that if declarer held the A Q of clubs, he would've finessed the Q long ago? I'll leave that up to you. I'm going to say no, for myself. In the moderate haste of play and given that we're not talking about experts here, I find West's "failure" to do so excusable and forgivable. There are so many more obvious errors -- e.g., the first two West's -- and this opportunity will present itself so rarely that I would not think it worth a word of criticism. If East chose to point out the opportunity, that's within the ballpark, but to be critical would not be in my opinion.
Case 4: Now it was East's turn to cough up that valuable fulfilling trick. Opening lead a low spade, won in dummy, a low diamond to the Q & Ace, another spade lead and now a low diamond taken by East with the J, with a switch to a club. Declarer took that trick with the Ace, cashed two rounds of trump, ending in dummy and now led the 10 of diamonds, covered by East and ruffed out by declarer. Now declarer went to dummy with a heart lead and led the 8 of diamonds, sluffing a club. Bid & made.
Case 5: This is only slightly different, but I'll give the play before discussing whether East was blameworthy or not: Opening heart lead, low to the Q of diamonds and Ace, shift to a spade by West, low diamond won by East with the J, followed by the Q of clubs, which declarer took with the Ace. Now a heart lead to the Q and a third round of diamonds, East going up with the K, is ruffed out. Declarer runs three rounds of trump, goes to dummy with a spade and cashes teh 10 of diamonds, sluffing a club loser.
Looking at the North and East hands, we see that declarer never gets a diamond winner if East will simply cover every diamond with the lowest possible card that covers. But East can't know that. Was he culpable of anything? Well, certainly nothing very obvious like the first two cases. But could he or should he have worked out the need not to go up? Or in other words, was it a situation an expert could've worked out? And I'm going to hafta say um-m-mm, no. Only hindsight tells us West has the 9 of diamonds. I don't see how he could've reasoned that this must be so, and hence, I find these totally excusable decisions.
Lemme go to Case 5, where East's task is easlier to see. After six tricks, the hand looked like this:
Declarer now led the 8 of diamonds, and East could've beat the contract by going low, as we can see. But I don't know how East could've been expected to know that. The situation depends on who has the 9 of diamonds, and who's to say that declarer couldn't have started with 3 diamonds (with the 9) and two clubs, where the defense could pick up 3 diamonds and a club.
When the Case 4 reached that point, declarer led the 10 of diamonds, smothering the 9, and there's nothing East can do about it if declarer guesses right. But even though the Case 5 East had an opportunity to beat the contract by placing the 9 in his partner's hand, I think only hindsight makes this a certainty.
A K 4
Q 10 7 
10 8 4 3
9 7 6