Whenever something is copied by hand, frailties of human eyesight enter in, particularly if that document is old and some ink has faded. Copying is also long, tedious work. It would take a scribe several months to copy just one Gospel. In some secular Greek manuscripts, scribes left a note at the end that indicates the patient labor involved: "As the traveler rejoices to see the home country, so the scribe rejoices to see the end of a manuscript!"
The invention of eyeglasses around 1375 certainly helped reduce the number of mistakes. And the invention of printing with movable type in 1456 assured production of duplicate copies. But prior to that, for over a thousand years, everything was done by hand, and the more times an ancient text was copied, the more chance for errors to creep in.
How reliable are the Greek and Hebrew manuscripts we have today?
The earlier copies are generally closer to the wording of the originals. The translators of the 1611 King James Bible, for instance, used Greek and Hebrew manuscripts from the twelfth and thirteenth centuries. Today Bible translators have access to Greek manuscripts from the third and fourth centuries and Hebrew manuscripts from the era of Jesus.
We even have the Rylands Papyrus, just a torn page with a few verses from John 18, that we can date between A.D. 100 and 150. So today we have access to a text of the Old and New Testaments that is more basic, more fundamental, less open to charges of scribal error or change.
Over the centuries of transmission, have scribal errors touched any core Christian doctrines?
No key doctrine of the Christian faith has been invalidated by textual uncertainty.
On the other hand, some passages have been affected. For example, take Mark 9:29. Jesus is explaining how he was able to cast out a demon, and in the earliest manuscripts, he is quoted as saying, "This kind can come out only by prayer." In the Greek manuscripts the KJV translators used, the two words and fasting are tacked on. I do not think that is an earth-shaking difference, but it is typical of the kind of changes we are talking about.
How would such a change have taken place?
By studying manuscript history, we see that the words and fasting were inserted between 300 and 600. These were the years of the desert fathers and the birth of monasticism. The number of official fast days was increasing, and the regimens of fasting were becoming stricter. Probably a devout scribe, himself part of a fasting tradition, believed that Jesus must have meant to include "and fasting," so he included the two words.
Some Bibles list three endings for the Gospel of Mark. How should we understand these?
The earliest Greek, Syriac, Coptic, Armenian, and Latin manuscripts end the Gospel of Mark at 16:8: "The women said nothing to anyone, for they were afraid." That does not sound like an appropriate ending for a book of good news, so some early scribes, undertaking their own research, added what they thought would be appropriate endings.
A few later manuscripts add just two or three verses to this abrupt ending, but most contain a longer ending, what we now number as verses 9 to 20. We can tell by examining the vocabulary that these endings were not written by Mark. So translators often put them in brackets or as a long footnote.
Why not just keep them out of the Bible?
Many translators, including myself, consider verses 9 through 20 to be a legitimate part of the New Testament.
In the third and fourth centuries, when church fathers were deciding which books should be included in the New Testament, these verses were already in the copies of Mark that most of them were using. In other words, the early church considered verses 9 through 20 to be a credible account of the Resurrection.