Suppose you are South with this unremarkable hand. To your great surprise, the auction explodes after your three-spade rebid, West saving in five hearts, then six. Partner takes the push to six spades, after which you might consider whether you should be bidding the grand slam. In a sense, your partner’s auction did not encourage that approach, so you pass.
Against your small slam, West leads the club king. You win with the ace and see that the slam looks solid. However, you should take the time to consider what might go wrong.
West must have a lot of extra shape for his eccentric bidding, so unless you know him to be the wildest of overbidders, you can expect him to have 12 cards in his two-suiter. (With 13, he might have done even more.)
If that is the case, spades could easily turn out to be 4-0. If you make the natural-looking play of the spade ace, you will be in trouble if West shows out. East will hold up his spade king until the third round and then cash some heart and club tricks. Equally, crossing to dummy in diamonds and running the spade jack also fails against accurate defense.
You must retain the spade ace in hand. The winning play is to lead a middling spade from hand at trick two. If East wins with the king, you take his return and draw trumps. If instead East ducks, you can cross to dummy in diamonds and lead the spade jack, making all the tricks.
Do you think that East might have taken the push to the seven-level here, with a double fit for partner’s suits and little defense? The spade king was likely to be well-positioned for declarer.