It is a key rule after trick one

Accurate defense often requires accurate carding by the defenders. In this deal from last month's World Youth Bridge Team Championships in China, it will be difficult for you to be objective, because you can see all 52 cards. However, here is what happened; which defender was at fault?

East's two-diamond opening showed a weak two-bid in either major. South, Zach Grossack of the USA1 junior (under 26) team, plunged into three notrump. Presumably, if he had gone more slowly, he and his partner, Oren Kriegel, would have reached the laydown fourheart contract (or collected 300 from four spades doubled). Three no-trump looked hopeless when West led the spade queen. (Some might have chosen the heart jack, thinking that rated to be partner's suit.)

Grossack took the first trick with his spade king, cashed his five heart tricks, then took the club finesse, which lost. West cashed the diamond ace before leading a low spade. East won with his ace and returned ... a diamond! South claimed an overtrick. Who was guilty?

At the other table, the English pair of Shahzaad Natt and Ben Norton had bid and made five clubs. They were unlucky to lose 1 international match point.

Against three no-trump, West clouded the water by cashing the diamond ace; he knew his partner had started with six spades, so he should have cashed the spade jack either at trick eight or after having taken the diamond ace. But East failed to remember that you lead the king from aceking after trick one. So, West's ace had denied the diamond king.