Metzger and Ehrman on Scribal Errors

The earliest copyists would not have been trained professionals who made copies for a living but simply literate members of a congregation who had the time and ability to do the job. Since most, if not all, of them would have been amateurs in the art of copying, a relatively large number of mistakes no doubt crept into their texts as they reproduced them. It is possible that after the original was placed in circulation it soon became lost or was destroyed, so all surviving copies conceivable have derived from one single, error-prone copy made in the early stages of the books' circulation. (Bruce M. Metzger and Bart D. Ehrman, The Text of the New Testament: Its Transmission, Corruption, and Restoration [4th ed.: New York: Oxford University Press], 24)