The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
John 7:53-8:11, The Pericope De Adultera
This passage is designated as the Pericope De Adultera, referring to the woman caught in the act of adultery. The passage is included in numerous uncials such as D05, G, H, K, M, U, and G. Among the minuscule or cursive manuscripts it is in 28, 700, 892, 1009, 1010, 1071, 1079, 1195, 1216, 1344, 1365, 1546, 1646, 2148, and 2174. Most Greek manuscripts contain this passage. It also is in early translations such as the Bohairic Coptic Version, the Syriac Palestinian Version and the Ethiopic Version, all of which date from the second to the sixth centuries. It is clearly the reading of the majority of the Old Latin manuscripts and Jerome's Latin Vulgate. The passage has patristic support: Didascalia (third century), Ambrosiaster (forth century), Ambrose (forth century), the Apostolic Constitutions (which are the largest liturgical collections of writings from Antioch Syria in about 380 AD), Jerome (420 AD), and Augustine (430 AD).
Most textual scholars consider the evidence against it to be overwhelming and reject the reading as original.  Yet, the passage still finds its way into the text of the majority of contemporary translations. Unlike John 5:4, which is confined to a footnote, this passage is retained in the text but usually separated with brackets (as with Mark 16:9-20). If the evidence against it is so convincing and the text is not considered genuine, should not this entire passage be removed from the text itself as other shorter passages are? If one is to remove smaller sections, would not consistency demand the same be done with larger sections if the amount of textual evidence is either the same or greater? Perhaps it is a matter of acceptance. Since this passage is beloved by the majority of the Bible reading public, to remove it from the text would be unthinkable.
Supporters of the Textus Receptus and the Majority Text, on the other hand, have soundly defended the authenticity of this passage.  The vast majority of all known Greek manuscripts contain this section. It is clearly part of the Traditional Text. Additionally, the internal evidence demonstrates that this passage is original. If we remove it we have a very erratic jump in textual thought.
The question arises as to why this passage was ever omitted. We find the answer in church history. Augustine makes an astounding statement concerning the authenticity of the passage. After citing the forgiving phrase of Christ, "Neither do I condemn thee: go, and sin no more," Augustine writes:
This proceeding, however, shocks the minds of some weak believers, or rather unbelievers and enemies of the Christian faith: inasmuch that, after (I suppose) of its giving their wives impunity of sinning, they struck out from their copies of the Gospel this that our Lord did in pardoning the woman taken in adultery: as if He granted leave of sinning, Who said, Go and sin no more! 
Augustine implies some fearful scribes who thought the inclusion might lead to adultery omitted this passage. This argument not only seems logical, but also consistent with human nature. It is, at least, as good as modern scholarship's view that the passage was added as a piece of oral tradition apart from inspiration. 
 Metzger, A Textual Commentary On The Greek New Testament, 187.
 See Hills, 150-159. John W. Burgon "Pericope De Adultera" ed. David Otis Fuller, Counterfeit Or Genuine (Grand Rapids: Grand Rapids International Publications, 1975) 133-158. Arthur L. Farstad, The New King James Version In The Great Tradition (Nashville: Thomas Nelson Publishers, 1989) 113-114.
 St. Augustine, De Conjug. Adult., II:6.
 Metzger, A Textual Commentary On The Greek New Testament, 188.