By Barbara Seagram 
Courtesy of BarbaraSeagram.com
On this auction, East led a trump. Declarer tried but could only manage to take nine tricks even though the diamond finesse worked. However North-South did not own the ♦ 10 so had to lose one diamond, two hearts and one club. Down one.
After the hand was over, they realized that while they could not make 4♠ , they could have made 3NT. So, they pondered, what went wrong. Just because you have a tool in your shed, it doesn’t mean that you have to use it, no matter what. South used Stayman just because he owned a 4-card major. He failed to consider that with a perfectly balanced hand and no ruffing values (short suits) that it would be easier to make nine tricks in 3NT than twn tricks in a major.
RULE: Do not use Stayman when you have a 4-3-3-3 hand.