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That portion of the New Testament was not included in the oldest manuscripts so it may have happened but is used so often (maybe I just remember it because I know it was added later) that I think it settles us down a bit and allows the Holy Spirit to speak to us.

I suspect it wasn't in John's original gospel account but that it did happen and some of the early Greek Christians decided he should have included it and they added it. I reckon they felt invited to include more by John himself, as the last verse in his gospel account was:

25 Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written. --John 21:25 NIV

Even though we have unusually good historical records on Jesus's life compared to other ancient individuals, I think its important to keep in mind, we likely only got a little of his teaching. We are very lucky to have the four gospels and letters of Paul all written when live witnesses were still around as sources. Of course copying records was rather expensive and likely many teachings that were written down during that time have been lost.