Source: http://www.qldbridge.com/gcc/2010/bulletin_02.pdf
South opened with a weak 2 bid and North raised to 4. Declarer takes the opening lead in dummy and leads the jack. Which card should East play? Answer: As West cannot have more than one card, East should play low, despite having the ten and nine.
The problem arose in the 1980 World Teams Olympiad. This was the trump position:
Declarer led the jack from dummy and there was good news and bad news. The bad news for the defenders was that East covered with the queen. South played the ace and now lost only two trump tricks and made the game. Had East played low, declarer would lose three tricks and the contract would have been defeated.
What is the good news? I was not in the East seat. Why did declarer lead the jack from dummy? If the suit divided 3-2, there would always be two losers. If the suit divided 4-1, playing the ace would always result in three trump losers no matter how the honours were divided. Leading the jack would hold the trump losers to two in these positions:
The jack is led from dummy. If East plays low, South lets the jack run and loses only two tricks. If East covers the jack South takes the ace and when the 9 or 10 falls from West, South will lose only two trump tricks.
Not only does the jack from dummy gain legitimately in these positions, but it also holds the trump losers to two if east mistakenly covers the jack with Q-10-9-3 or K-10-9-3.