Source: Spotlight on Card Play by Robert Darvas and Paul Lukacs
You are South, and your contract is Three No-Trumps. West leads the 
6.
IS THE CONTRACT IN SIGHT? STOP AND THINK
Yes. You have two tricks in Hearts, three in Diamonds, and four in Clubs, totalling nine.
IN WHAT ORDER DO YOU INTEND TO BRING HOME YOUR TRICKS? STOP AND THINK
It looks a good idea to play low in dummy so as to take the trick cheaply in South’s hand as the Eleven Rule tells you East has no Heart higher than the Six. That would be all right if you could be certain that West has led his fourth highest Heart. But perhaps he hasn’t; and then the Rule of Eleven simply won’t work, and you may well find your K forced out by the Knave or Queen appearing from East’s hand. You next cash South’s three high Clubs, enter dummy with a Diamond—for, of course, opponents hold up the Ace—and cash the 
A. Again you play Diamonds, and the Ace appears only on the third round of the suit. Now there is no entry in South’s hand for making the third Diamond trick. You can no longer get nine tricks.
CAN YOU SPOT THE MISTAKE? STOP AND THINK
The first trick was taken in the wrong hand. You must resist the temptation to assume that West’s 6 is the orthodox fourth-high lead. In this hand you dare not bank on the Rule of Eleven, and risk having to play your K prematurely. So you take the first trick with dummy’s A, and thus ensure the safety of your K as entry to your hand for the third essential Diamond trick after you have cashed your four Club tricks in the way already described. That makes your contract nearly a certainty.
WHY ONLY NEARLY? IS THERE A DISTRIBUTION OF THE OPPOSING CARDS THAT CAN DEFEAT YOU?
STOP AND THINK
There is. If either opponent has six Clubs and a convenient entry, declarer can lose two Clubs, two Spades, and one Diamond. The probability of this is less than one per cent. But suppose it does happen that either opponent fails to follow to the first round of Clubs.
WHAT DO YOU DO ABOUT IT? STOP AND THINK
Nothing. You simply play on as planned. For in spite of the 6-0 Club break there are still some chances in your favour.
WHAT ARE THEY? STOP AND THINK
(1) The 
A is singleton or doubleton. (2) The defence slips up by taking the first or second round of Diamonds. (3) The defence makes some other mistake. (4) The opponent with the Clubs has no entry. (5) Spades are blocked.