When a player leaps into a slam contract without trying a Blackwood four-no-trump bid to check on partner's aces it most likely means he has a void suit and aces don't really concern him. Against such a kangaroo type of bid, defender would have to be an ostrich to expect two aces to win tricks. So West has little excuse for leading the ace of hearts in the deal below. But what should he lead? Look only at the West cards, study the bidding and decide what you would have led.
Opening the heart ace would make the slam a romp. Dummy trumps and declarer knocks out the spade ace. Since his heart king is now high, once trumps are drawn he can discard his remaining heart loser on the club queen.
The singleton club lead gives declarer four sure club tricks. A little thought should steer West away from this choice. If East can get the lead, the ace of trumps will be the setting trick anyway and West need not risk helping declarer play the club suit correctly.
West made a better choice when he led the ace of spades and followed up with a second spade to cut dummy's ruffing power. This left dummy with only one trump. Declarer could now ruff only one heart and discard one on the queen of clubs, but he needed to set up another trick to take care of his third heart. He elected to try to use dummy's fifth club for that purpose, but when he played the king and another club, West showed out and that ended that hope.
South tried to recover. He cashed dummy's top diamonds and ruffed a third round. But the suit failed to split, and dummy had only one remaining entry—the trump, that would take care of one heart loser. This left South one trick short of his slam.
It was hard luck to find the clubs stacked against him and, mathematically, there was just as much chance that the missing diamonds would misbehave. Nevertheless, South was entirely responsible for his own downfall, for he could have given himself two chances instead of one simply by trying out the diamonds first.
With the trumps drawn, dummy's top diamonds are cashed. When both opponents follow, the contract is assured. South ruffs a diamond, plays the king and ace of clubs, ruffs another diamond, gets back to dummy by trumping a heart and discards the remaining hearts on the good diamond and the club queen.
Now suppose East had five diamonds instead of five clubs. When West shows out on the second diamond, there is still time for declarer to cash the club king, lead to the club ace, discard on the club queen, ruff a fourth club, trump a heart to get back to dummy and discard the last heart on the established club winner.
So it turns out that the best lead against the slam is a small heart. Could this have been foreseen? Yes. The bidding has told West that dummy must be void, but it hasn't told South who holds the heart ace. Declarer could still make the hand by letting the lead run to his heart king, or by taking a first-round finesse of the club 10. However, the chances certainly are that declarer would ruff the opening heart lead in dummy and play one round of trumps. Then, after a trump continuation, he would probably misguess the club suit.