When Declarer leads a trump from Dummy at Trick 3, East might casually play the King!  What an extraordinary play!  Now look at the hand from Declarer’s point of view.  East “obviously” started with the singleton ♣K … the bad news is that there is a trump loser to West’s ♣J85 … the good news is that by now playing low towards Dummy’s ♣T (instead of cashing the ♣Q) Declarer can force a second entry to the board for the Diamond finesse.

So, Declarer tries the aforementioned, but Dummy’s ♣T is taken by East’s Jack!  That play of the ♣K has swindled a trump trick for the defense.  And it has also swindled Declarer out of an entry to the board.  Now the Diamond finesse cannot be taken and it’s down one instead of making with an overtrick!

East’s falsecard was almost a free shot.  East may not have foreseen the havoc it would wreak, he just needed to know that unless West had the singleton ♣A his play would not cost, and that one way or another it might gain.  As for South, he could have saved the day.  He had ruffed the opening Heart lead with the trump nearest to his left thumb, and unfortunately for him that was the Two.  If he had saved that valuable Two for later and splurged out with the Six or Seven, he would still have a trump entry to the board for the Diamond finesse.  But who would think of that at Trick 1?