A capable defender operates like this: He visualizes a hand for declarer, consistent with the bidding and play, that will let the defense prevail. Then he proceeds on the basis of his assumption.
If declarer has a different hand, and a different defense would have worked - or maybe no defense would have worked - that's bad luck. But if the defender's picture of declarer's hand was unreasonable, then he goofed.
In today's deal, West led the deuce of spades against four hearts, and East captured dummy's king and mulled over his next play. Neither a spade continuation nor a club shift looked promising, so East tried a diamond.
South won in dummy and let the queen of hearts ride. When West took the king, he shifted to a club, and East took the ace and led another diamond, hoping West could ruff. He couldn't, and South drew trumps and easily took the rest.
Was East's defense an error? An unlucky decision? How would you have defended?
East was assuming West had a singleton diamond, but West was known to have a weak hand. Assuming he had an entry in trumps (necessary if the defense was to have any chance), he would surely have led a singleton.