Largesse

J 2
K 8 7 4
A 6 4 2
K J 8
10 9 7 3 A K 8 4
6 5 Q 9 3 2
Q 9 8 7 5
6 5 4 A Q 10 7
Q 6 5 West North East South
A J 10 P 1 Dbl Rdbl
K J 10 3 P P 1 1 NT
9 3 2 All pass

The day after I found a couple of illustrations on an OKbridge tournament of 4-card carelessness before I had hardly started to search, I was the recipient of a similar largesse myself. Here is the hand. The bidding practically determines how to play the hand.
Opening lead was the 10 of spades, won by East who cashed his second top honor, then continued with a third round. East's bid indicates all the outstanding strength, except in diamonds where it suggests shortness. So my first step was to cash the king of diamonds, finesse the jack,and then low to the ace, as East sluffed first a heart, then a club. I now led a low heart to the jack, cashed the ace and overtook the 10 with the king, felling the queen, which allowed me a fourth heart trick and 8th overall, and of course, I was finished at that point.
Should East have been able to figure out that the heart was wrong? Oh, I think that's hard to say. And yet, he can see four hearts in dummy, and that's a pretty good warning that the suit might go to the fourth round. It doesn't look as though he's going to get two leads through dummy's clubs, and so he might reason that he can get down to A Q tight. East can't know who has the 9 of spades. If his partner does, then hanging onto the heart length is the thing to do. In that case, he has a means of ensuring that he'll get a lead through the K J of clubs. But if declarer has that 9, then East is substantially squeezed on the third round of diamonds: If he discards a second club to protect his heart length, declarer can cash his spade (after the heart finesse), run hearts to the fourth round, East winning and being forced to lead clubs himself. And if he sluffs the heart, as he did, he gives up a heart trick, but will eventually get two clubs.
Which is pretty close to saying that if declarer has the high spade, it doesn't matter what East does (provided declarer knows what he's doing!), whereas if West has the high spade, then it does matter what East does, and he must keep his heart length. Which is a no-brainer if you have time to think it through -- heads I win, tails it doesn't matter what I do -- but not a line of reasoning that leaps out at the defender.
How serious was that discard of the heart? Well, the final tally shows plus 120 worth 88.73, plus 90 picking up 70.10. Not a great number, but perhaps a significant difference for one careless discard, no?
This was one of my first entries on this web site. Some time after entering it, I initiated a new category entitled Same q.v., where I argue that by and large, you'll want to keep the same length as declarer is showing (or is presumed to have in the closed hand). And that doesn't take a long chain of reasoning.