A poisoned chalice is something that seems good at first (a beautiful cup filled with delectable wine) but after the person accepts the cup and drinks from it he or she discovers that it is poisoned.
The Lady or the Tiger is a particularly clever and distracting tactic – it is also one of the most unethical. It has been around for centuries but is well described in Frank Stockton’s short story by the same name – The Lady or the Tiger.