In my copy of the NIV Bible, there is a footnote between John 7: 52 and John 7:53 that states, ” The earliest and most reliable manuscripts do not have John 7:53 – 8:11.” Of course that is one of the most cherished stories that illustrates the loving and forgiving character of Jesus in his defense of the woman caught in adultery while he skillfully exposes the self righteousness of the Pharisees.
I have read a commentary that suggests the responsibility of the added passage rests on the shoulders of an ambitious scribe that wished to embellish (as if he needed to) the account of Jesus’ ministry. If this is true and the more reliable manuscripts do not include these verses, why then does it appear in modern copies of the Bible? It seems that the inclusion of this questionable passage might possibly raise an issue with the veracity of the rest of the Book of John.
Although most early manuscripts don’t contain John 7:53-8:11 (the Woman Caught In Adultery) the consensus, even among the most vocal critics of it’s placement in John, has always been that it’s an historically authentic account from the life of Jesus that was included in a number of other reliable contemporary non-Biblical texts. The real question is not whether it belongs in the Bible, but whether it belongs in John’s gospel. Since later manuscripts show it in John, it’s been left there, with some translations carrying the notation you cited. The most popular alternative placement is after Luke 21:38.
As for why it was left out of most early manuscripts, there is no reliable explanation.