There's a difference between wishful thinking, in which we all indulge occasionally, and "thoughtful wishing." When you need the defenders' cards to lie a certain way to make your contract, assume that they do. If your contract is in danger only if they lie a certain way, assume that they do - and take what precautions you can.
In today's deal, South arrived at four spades after East-West had bid and raised hearts with featherweight hands. South ruffed the second heart (not best defense), drew trumps, and led a club to dummy's king. He was surprised when East took the ace and returned the jack to the queen.
South next let the queen of diamonds ride. Predictably, West took the king and led a club to East's 10 for down one.
"Swap the positions of the ace of clubs and king of diamonds," declarer shrugged, "and I make an overtrick."
South was guilty of wishful thinking. Instead of hoping for an ideal lie of the cards, he must assume a lie (the actual lie, in fact) that puts his contract at risk.
After South draws trumps, he should lead the ace and a low diamond. If East had the king - a finesse would have won - South would be safe as West would be marked with the ace of clubs for his opening bid. South could lead twice toward dummy's K-Q of clubs, losing a club, a diamond and a spade.