Housekeeping I

K Q 8
K Q 10 8 3
A 5 3
J 9
5 4 3 J 10 6 2
7 4 J 9 5
J 9 7 2 10 8 4
7 5 4 2 A 10 3
A 9 7
A 6 2
K Q 6 Contract: 6 hearts or NT
K Q 8 6 Opening lead: 9 of diamonds

This was one of the more elementary cases of a failure to do some housekeeping before cashing out. Actually, the first "down one" I looked at was played in such a bizarre manner I wasn't even going to include this: In a 6 no contract, the opening diamond lead taken in dummy, a club was led to the king, establishing a 12th winner -- if hearts break evenly (as they do): that's five hearts, three spades, three diamonds and one club. The club has to be developed, of course. Anyway, after a club to the king, the queen was played(!) necessarily gobbling up the jack, allowing a doubtless surprised East to take the trick with the A and cash the 10.
Declarer of course (once he'd established a club winner), should test hearts for a favorable break by going low to the K and back to the A. This allows him to pick up the whole suit on an unfavorable 4-1 break if West has the J, while there's not much he can do if East has J 9 low low. Anyway, once he has hearts down pat on the 3-2 split, he's done all he needs to do to guarantee 12 winners and might give a try at picking up a 13th trick (in clubs) from an inattentive defender. And if the A is cashed, well, declarer would be on claim for his contract without overtricks. But it wasn't to be.
Another declarer misplayed the hand in a more common manner, to wit:
Diamond to the king, heart to the king, heart to the ace (establishing the 3-2 split in that suit), heart to the queen, spade to the ace -- are you getting a little nervous by now? I mean, declarer has to develop a club winner! You don't wanna fool around with spades or diamonds. They're already looking at you with three top winners each. Three plus three plus five. That's 11. You need one more. It's advertising itself. It can only be a club. Now, you'll quite likely have to lose the lead once to get it. Okay?
So you don't wanna use up all your spades before losing the lead (i.e., particularly in no trump), and you don't wanna use up all your diamonds either. Don't you think you'd better establish that 12th winner (as soon as trump are drawn in a heart contract)?
But that wasn't what happened. Spade to the king. If I were dummy watching, I'd be very nervous by now. And then, yes, the queen of spades was cashed!
By chance, the ace of clubs lay with the long spades, so there was no way to make the hand now (in no trump, or in hearts if they've been run). At least the declarer had the saving grace to call herself a "low intermediate". Now, if only she'd cashed the top diamonds first, instead of spades, for the long diamonds don't have the A of clubs. Actually, she did run her diamonds before hitting clubs, though that had no effect on the hand. But running the top spades before doing the housekeeping she had to do was a killer.