The Lewiston Daily Sun – 9 Sep 1970
If you could be a good bridge player by just following general rules, there would be 40 million experts in the United States alone, and they would generate enough hot air to melt the polar ice cap. Fortunately for our ecology, bridge isn’t that easy, and there aren’t that many experts. Today’s hand isn’t very hard, but it shows how those who follow rules blindly go astray.
South Dealer E/W Vulnerable
Opening lead — 
Q
Dummy’s ace of hearts won the first trick, and declarer naturally led a low trump from the dummy. East played his low trump because he had been taught to play «second hand: low ‘.
South won with the king of spades, cashed the king of hearts to discard one of dummy’s diamonds and then tried a finesse with the ten of clubs. The defenders could take only one club, one diamond and one trump: and South mule his game contract.
Perhaps the comments have helped you see what went wrong, but more probably you saw the error immediately. This was no time for East to play second hand low.
King Was Marked
The opening lead of the queen of hearts made it clear that West didn’t have the king. If West’s hearts had been headed by the king and queen he would have led the king instead of the queen. Therefore East should have known that South held the king of hearts.
East could foresee that South would discard one of dummy’s diamonds on the king of hearts, if he could get to his hand quickly and it was up to East to act even more quickly to prevent the discard. There is always some risk in playing the ace of trumps in second position: your partner may have the singleton king or queen. But failure to act may be even riskier. East should have stepped up with the ace of trumps in order to lead a diamond at once. West would get two diamond tricks, and South would go down.