West leads a high club. South wins in hand and, wishing to draw trumps quickly to stop the possible club ruff, plays a spade to the king. That holds, East playing the jack. Now South plays another trump off dummy and the roof falls in. East discards and West pulls out all the remaining trumps. Four diamonds follow, and declarer ends up 2 down.
South's mistake was not counting his losers. He could afford to lose 2 diamonds and 3 trumps, so all he needed to do was to ruff a heart in dummy and play on clubs, making 2 with ease. This gives up on the overtrick, but ensures +110.
This play is very simple, of course, but it's easy to miss if you don't keep your eyes on the ball

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West North East South
.................pass..pass
pass...1♣....pass...1♠
pass..pass....dbl...1NT
pass...2♠ (all pass)