See the diagram on the right. White has serious problems. His bishop on f4 is attacked by the knight and with it the pawn on e5. To make matters worse, black has a well supported passed pawn on d4. So what to do?
White decided in favor of drastic measures and played 1. Be3. His idea is: if you take my pawn on e5, I will retaliate with the capture on d4. If this exchange would materialize, it is a certain draw.
The problem is: white overlooked a tiny detail. Do you see this ‘detail’?