Playing five-card majors, South opened 14 and North forced to game with 2NT (agreeing spades). South had several options open to him at this point but you must agree with his choice of 44 ("Yes, I know that you have forced to game, but I have not the slightest interest in a slam.") Indeed, having decided to open on that garbage, I would almost have been tempted to break all the rules.
So there was South in Four Spades, and West led #K. Clearly there was no rush and, when declarer held off, West switched to 2K. Following a sensible plan, declarer won, played off three rounds of hearts (trumping the third), cashed #A, and ruffed a diamond in dummy. Having eliminated the red suits, he got off lead with a losing club and prayed for a blocked position in the suit.
In a sense, his prayers were answered. West duly won with his queen but held no more clubs. Faced with the choice of opening up the trump suit or conceding a ruff and discard, he rather forlornly continued with a fourth round of diamonds. Dummy ruffed, away went declarer's losing club, and now there was only a trump trick to be lost.
But suppose that West had got off lead with a trump. Fine, declarer wins cheaply and cashes 4A (leaving West with the singleton queen). The trouble is that although 4Q is due to fall, dummy has nothing but clubs left. East wins and, whatever his partner discards, has just the right card to promote 4Q for the setting trick.