You're Not Going to Believe This

K
Q 7 6 2
K Q 9
K Q 6 5 4
A 9 8 5 4 J 10 6 3
J 3 9 8 5 4
8 6 5 4 2 J 10 3
2 9 3
Q 7 2
A K 10
A 7Contract: 6 no
A J 10 8 7Opening lead: 2 of diamonds

Assuming declarer doesn't play some fancy-schmancy game of "unblocking" the K or Q of clubs under the ace, how many entries to the closed hand does she have? Of course the answer is six. Now: after trick six, how many entries to the closed hand do you suppose declarer had? Ah, some of you guessed right, maybe from the title above. That's right. She had no entries to the closed hand. She made a beeline to eliminate them all. Ace of diamonds on the opening lead, reasonable enough. A of clubs, J of clubs, 10 of clubs. Not so reasonable. A K of hearts! Totally unreasonable!
Just think. If hearts had split 3-3, she'd have come out even with people who played reasonably! But hearts didn't split 3-3 and after losing a trick to the A of spades at trick 9, she was forced to lose trick 13 to the 9 of hearts! What can one say? Trick two should have been a low spade to the K. And I dare say would have been had this declarer simply counted 11 top winners with one that so obviously could have been developed on a low spade to the K.
I can only say again (and again), count first, and then develop. Oh, there'll be hands where there's really nothing you can develop and hands where you've got 12 top tricks and might as well squeeze for a 13th. But you know what I mean. Almost always. Think develop.
[Years later] If declarer led the A K of hearts right away and the J falls, declarer would obviously have four heart winners by cashing the 10 first and now entries to the closed hand wouldn't be so important, since declarer could rattle off 12 top tricks, needing only one entry to dummy outside of the heart suit. I don't know why I made no reference to that when this was first entered. As for losing the final trick to the 9 of hearts, it would seem that declarer, after wiping out entries to dummy, later overtook the 10 of hearts, looking for a 3-3 split, which was not to be.