There’s just no accounting for the kind of mistakes defenders sometimes make. Chalk it up to fatigue, inattention, mental aberration — whatever the case, there’s no denying that it has happened to all of us.

Take this case where South was in six hearts and West led the ace of clubs. When East dropped the ten, West continued with a club, felling East’s king as declarer ruffed in dummy.

At this stage, it would seem that with normal play declarer is headed for down one. Unless he can see through the backs of the cards, he will cash the queen of hearts and lead a heart to the ace as East follows with the deuce and seven. When the trumps divide 4-1, South has to lose a trump trick, and that’s that.

But a funny thing happened on the way to down one. When declarer led the eight of hearts from dummy after cashing the queen, East covered with the ten! Perhaps East thought he was guaranteeing himself a trump trick with this play, but it proved to be just the opening South needed.

After winning the ten with the king and seeing West show out, declarer realized that the only way a trump loser could be avoided was by means of a coup. This necessitated reducing his trump length to that of East. Accordingly, he next led a diamond to the ace and ruffed a diamond with the six, leaving him with the A-9 of trump.

Dummy was entered with a spade, and the king of diamonds was cashed, declarer discarding the queen of spades. Spades were then run, and East’s “sure” trump trick went up in smoke. Whether he trumped a spade at some point or discarded instead, his J-7 of trump were sure to succumb to South’s A-9, and the slam was home.