We're having a good conversation about John, and I haven't responded yet to all of the comments or discussion, but I will do that in the next few days.
John 8 is most known for the opening 11 verses. Depending on the translation of the Bible you have, you will have some kind of note like: "The earliest manuscripts and other ancient witnesses do not have John 7:53-8:11."
This requires a much more in depth discussion, but let me attempt to put it into a concise few statements. At the heart of the matter is this question: How did we get the Bible we have today?
1) The Bible is a compilation of many different parts. For the New Testament, there are many letters and historical accounts. Each was written at a different time, in a different place, on a different piece of papyrus.
2) We do not have any original pieces of those parts. The earliest pieces we have were copies of copies. These weren't scanned or xeroxed. They were hand copied.
3) In some cases, we have multiple fragments of the same letter or account. Mostly, they are the same. Sometimes, they are different. Which one is the 'right' one? Which one reads like the original?
At the heart of this, we want to know 2 things:
1) What John said. If someone else added to it later, it's not part of what John originally said.
2) What God wants us to know. This is more important than what John originally said or didn't say. But the it's much more subjective.
What this note in your Bible means is that John's original Gospel probably did not include this account of the woman caught in the act of adultery. Does that mean it didn't happen? NO. It simply means it probably wasn't in the original manuscript that John wrote.
This is NOT the only time this happens. We've already noted it in 5:3 (same thing). You can see another example in Matthew 18:11 and in what we call the 'second ending of Mark', where the earliest manuscripts end Mark with v.8, but others keep going.
So, what are we to do with these texts?
I believe that none of them are inconsistent with the message that Jesus gave us or the word God wants to give to us now. I have taught about John 8:1-11 many times and I personally believe it's a story that really happened.
But you shouldn't proceed in ignorance: there are some texts in the Bible that are worth looking at with extra effort. What we want to know is what God wants us to know. No more and no less.
John 8 doesn't end with v. 11. It continues with some very rich teachings. Note especially v.58. when Jesus says "before Abraham was born, I am!", he is clearly communicating his status as God. It would be worthwhile for you to look at Exodus 3:13-14. These verses would be well known by the Pharisees and other faithful Jews, and they would immediately come to mind as Jesus spoke those words. They knew exactly what he was implying.