Trading Mistakes

A J 6 4 3
A 10 5 3
K 10
A 5
8 2 K Q 10 9 7
K Q 9 2 8 6 4
5 A 6 2
Q J 7 6 4 2 10 9
5
J 7
Q J 9 8 7 4 3 Vul: Both
K 8 3 Opening lead: 4 of clubs

SouthWestNorthEast
Pass Pass 1 Pass
1 NT 2 2 Pass
3 Pass 3 NT All pass

Opening lead was a low club, which declarer let ride to his king. Now a diamond to the king brought the ace, another club wiped out declarer's last club stopper, but that was no problem. Declarer overtook the 10 of diamonds and ran 10 tricks for a pretty good board. You can doubtless see how the sides traded mistakes.
You don't want to let that opening lead ride to your king, which an awful lot of players do in a lazy manner. When dummy comes down, you've got to see where you have entries and where you're going to need entries. And you'll want to preserve entries, if at all possible in the hand where you're going to need entries. Okay? You've only got one certain entry to the closed hand. And you want to preserve that entry whatever else you do. Nor does it seem as though you'll be needing to preserve entries to dummy. So take the opening lead with the ace, and now you can hit the king of diamonds, overtaking the 10 if you don't draw the ace and continuing the suit.
Declarer got a bit of a break on that opening lead. The 8 of spades lead, for instance, could have been more difficult to handle. But he offered that gift right back to the defense by letting the lead wipe out his certain entry.
East, however, came to the rescue in capturing the king of diamonds. Now it may be tempting to capture the king. That's what aces are for, we're told. But there's an asterisk there. The asterisk says sometimes or usually or by and large. Here the bidding should have told East almost exactly what declarer had in is hand.
South originally bid a no trump, showing a not very robust hand in hcp's, and then at the three level, no less, he introduces a suit, which can only bespeak a long suit. If that's not enough, he immediately goes after the suit. Does that look like a guy who has no honors in diamonds? C'mon. You've gotta know that declarer has the diamond suit wrapped up and that barring an improbable king of hearts, his only entry to it can be the second round of the suit, and that capturing that card has to be more important than capturing a king.