What was the specific trap being set for Jesus by the Pharisees in John 8?

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)](http://www.biblegateway.com/passage/?search=John%208:2-6&version=NIV) What specifically was the trap they were trying to get Jesus to fall into?