Wow, 5 tricks lost in 5♣. So I take it spades are 2-2 and diamonds 4-4? Because declarer would have to lose 2 spades, 2 diamonds and a heart to go down 3. If my inferences are correct, that means: (1) In 5♣, the declarer failed to take a safety play in diamonds to avoid the loss of 2 tricks. (2) North is probably 2-1-4-6, and I'm willing to bet that that singleton is the K♥. No matter, dropping the singleton K♥ is still quite against the odds. If that's how the making slam was brought home, that declarer either misplayed the hand and got lucky, or was "swinging." (I suppose, if the hand was played in person, the "table action" might have suggested the blank K♥.) Show us the full hands when you're ready :)