I failed to concentrate on what could go wrong on this one. And what could go wrong did go wrong!
Put yourself in the South seat and see if you can do better:
Opening lead was the five of hearts. East won the ace and returned the four, I won with the king, and West played the six. I then cashed the ace and king of spades and saw West discard the nine of hearts on the second spade.
From here is looks easy: Cash the queen of hearts to discard a diamond and draw the last trump. Losers are one heart, one diamond, and one club.
Oops, there was another loser. What went wrong, and how could I have avoided the error?