The teachers of the law and the Pharisees brought in a woman caught in adultery. They made her stand before the group and said to Jesus, "Teacher, this woman was caught in the act of adultery. In the Law Moses commanded us to stone such women. Now what do you say?" They were using this question as a trap, in order to have a basis for accusing him.
But Jesus bent down and started to write on the ground with his finger. When they kept on questioning him, he straightened up and said to them, "If any one of you is without sin, let him be the first to throw a stone at her." Again he stooped down and wrote on the ground. At this, those who heard began to go away one at a time, the older ones first, until only Jesus was left, with the woman still standing there.
Jesus straightened up and asked her, "Woman, where are they? Has no one condemned you?" "No one, sir," she said. "Then neither do I condemn you," Jesus declared. "Go now and leave your life of sin."
I suspect the most surprising thing people learned from Bart Ehrman's Misquoting Jesus was that the story of the woman caught in adultery doesn't belong in the Bible. Apologists are quick to point out that textual critics have long known that this passage does not appear in the oldest and best manuscripts and that it is not written in the same style as the rest of John's gospel. They are equally quick to point out that removing it does not threaten any essential doctrines of the(ir) faith. Still, the average lay person did not know this because no pastor ever explained it when he discussed the passage in a sermon. It was probably pretty jarring for a lot of people to learn that such a wonderful story was not authentic.
It is natural to wonder why the scribes added that story to John's gospel, but the answer seems quite obvious: IT'S A GREAT STORY!! The story epitomizes the characteristics and qualities that make Jesus such a compelling figure and anyone familiar with it could not help but want to see it preserved for all people to hear. As a result, scribes familiar with the story tried to find a place to put it. Some inserted the story after John 21:25 while others found a place for it after Luke 21:38. The fact that scribes did so poses a challenge to the orthodox evangelical understanding of the gospels.
It is not that scribal changes undermines the doctrine of inerrancy because evangelicals believe that this doctrine only applies to the original writings, not to their transmission. The problem is that the scribes felt free to add a story simply because it was such a great story. If the scribes truly believed that John's gospel was the personal eyewitness account of one of Jesus' disciples, would they have had the temerity to add a story just because they liked it? Doesn't their willingness to do so suggest that they understood the gospels to be collections of stories taken from oral tradition by anonymous authors, which could be improved by the inclusion of other stories found in oral tradition? Doesn't it show that the historical pedigree of a story was not as important as its power to inspire and illuminate?
If scribes included stories based on their understanding of Jesus' teachings and character rather than the story's historical pedigree, is there any reason to think that the original writers did not do so as well? Moreover, is there any reason to think that stories might not have been omitted without regard to historical authenticity if the story did not fit the scribe's or author's understanding of who Jesus was and what his life meant? Once that possibility is allowed, you have to allow for the possibility that the oral transmission of the stories prior to the composition of the gospels may have included a series of additions and subtractions based on each person's understanding of Jesus.