David could have lost to Goliath
This illustrates once again how small details make a big difference.
First, the original problem published in the German magazine "Die Zeit".
White to play and win
And now a very similar position, but this time, the previous idea doesn't work.
White to play and lose 
Let's say White takes the Queen, which is not the best move... Can you spot the difference?
Take the Black stones and win:
