The notion that the Bible contains the inerrant word of God is a 20th century notion. Perhaps the ease with which we produce identical copies of books in our time has fed the mistaken belief that book-copying throughout history was or even could be without error.
The earliest copiers of materials that became our Second (New) Testament were first copied by amateurs who were often barely literate themselves. I got the image of someone in a church trying to restore a stained glass window without any training. The errors would be obvious. Imagine being unable to really read yourself, but having to transcribe gospels or letters that have been copied a hundred times already. You certainly wouldn't be working with a perfect copy ... or no duplicate would be needed, right? Or perhaps someone would be waiting to take it to a far-off church and you'd need to hurry to get it done.
Later, scribes were part of never-ending church politics and made tiny adjustments to either correct for what they saw as bad theology, or to correct for what they thought were mistakes by previous scribes. What would you do, as a professional, if you think someone before you made a mistake? You'd try to correct it! But wouldn't you sometimes probably 'correct' accuracies, out of error yourself?
In retrospect, some of the alterations we are certain were made are really quite shocking.
This is a peculiar book to recommend. Although Ehrman clearly knows what he's talking about, I think he inches along a bit too slowly and repeats himself a little too much. The result is a book that's probably too difficult for friends/family who have been bitten by the evangelizing bug to take the time to understand but probably a little too unacademic for my friends from seminary.