Throughout the game, especially in the Viper Manor, there are many decoy treasure chest which are actually enemies. When you encounter them they tell you, you only have "one chance" to attack the correct one to recieve the prize they have. All you have to do to ensure yourself the prize is to randomly attack one. If you do not choose the right one the first time, just "run away" and repeat. You can do this as many times as you want at no cost.