Because you played on ♦ in stead of on ♥ (that was probably bad play); you reach the following end position:
You already lost a ♦ and a ♠ trick. You just played a ♦ to the K in North to discover that ♦'s break 4-1. The lead is in North (dummy) and you can loose one more trick. The ♥ position is not really known, so I gave both opps the ♥K.
After all it just felt great to play the right card.
I couldn't resist to post this.
What card do you play?

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