west	north	east	south
	-	-	Pass	1♠
	Pass	2NT*	Pass	4♠
	All Pass

Opening lead (at both tables): Queen of Hearts
East-West were Vulnerable

A few years ago, a four spade contract played at both tables decided the winner of a bracketed knockout event.
 
Having heard partner's pass, both west players decided to stay out of the auction over south's opening. Both north players used a Jacoby 2NT bid to force to game, and both south players signed off.
 
Both west players led the queen of hearts, won in the closed hand. and both souths cashed one high trump. A club was led to the king, which held, and a 2nd trump was won with the queen. Another club lead went to west's ace, who then exited with the jack of clubs to dummy as south pitched a diamond.
 
Play had been identical at the tables to this point, but it was at this point that the lines of play diverged. Before reading on, you may want to consider what you would do next.
 
Our first declarer had noted the location of the high cards that had fallen, and reasoned as follows:

"West has already shown up with the ace and jack of clubs, and appears to have the queen and jack of hearts for his opening lead. Holding the ace of diamonds would give him twelve high card points, and it's likely he would have doubled or overcalled if such was the case.

Believing the ace of diamonds was on his right, south pulled west's last trump, played a heart to dummy's ace. The three of diamonds came next. East played low, and south put up his king, losing to west's ace. West exited with a heart that declarer could ruff in hand, but south eventually lost two more diamonds. Down one.