Source: http://www.qldbridge.com/gcc/2010/bulletin_08.pdf
When the opponents leap to slam it is easy to believe them and take a sacrifice. It can be very profitable if you are right but the question is, can you trust your opponents? Today’s hands offer advice on when to trust the opponents.
You hold as South: Dealer East, East-West vulnerable
The bidding goes, one diamond by East, pass by you, three diamonds by West, which is forcing to game, three spades by partner and six diamonds by East. Wow, that came fast. Do you sacrifice in six spades? The penalty will surely be worth less than the value of their slam, which you assume will be making. This deal comes from the 2001 US Team Trials. Fred Stewart bid six spades with the South cards, which was five down for -1100. This would have been a good trade if the opponents’ slam had been making but it wasn’t. Eleven tricks is their limit, leaving Stewart to rue his decision. If you bid six spades, be comforted by the fact that a majority of world- class experts who were polled about the hand also bid six spades. It feels better to be in good company when you do the wrong thing! The whole hand:
Similarly…You are North, dealer East, East-West vulnerable
East opens one club, partner overcalls one diamond, West bids two hearts, which shows a good hand with hearts and club support, and you bid two spades. East, five clubs, partner bids five spades and West calmly bids six clubs. Do you sacrifice in six spades? This deal comes from the final of the Grand Prix event played in Salt Lake City. The Poles were neck and neck with the Canadians at the time. The right decision from North would put Poland back in the lead. 
North for Poland was Jacek Pcszcola (affectionately known as Pepsi). When the Canadians bid to six clubs, Pepsi bid six spades. Not surprisingly he played the hand well to hold to two down to give 300 points to Canada. East, Joey Silver, led a heart, otherwise six spades would have made, and West Gitelman took three hearts and played a fourth heart. Pepsi might have played safe for three off by pitching a diamond but he correctly ruffed with the jack of spades. Thereafter, he took successful finesses in spades and diamonds to make the rest. Be that as it may – six clubs was failing, being off two aces! Poland ended up losing narrowly. In the modern game, players bid more and more with big-fitting hands. Their often-heard logic is that they are taking insurance. But bridge is like poker and the smart new hustlers are overbidding to extract more from their insurance-minded opponents. That’s what East did on the first deal and West did on the second. So when can you trust an opponent who leaps to slam?
The answer is never! The truth is that it is hard to make twelve of the thirteen tricks, so make them work for their points.