In the story of the woman caught in adultery, John indicates that the question of the Pharisees was a "trap".
2 At dawn he appeared again in the temple courts, where all the people gathered around him, and he sat down to teach them. 3 The teachers of the law and the Pharisees brought in a woman caught in adultery. They made her stand before the group 4 and said to Jesus, “Teacher, this woman was caught in the act of adultery. 5 In the Law Moses commanded us to stone such women. Now what do you say?” 6 They were using this question as a trap, in order to have a basis for accusing him.
John 8:2-6 (NIV)
What specifically was the trap they were trying to get Jesus to fall into?