In the absence of printing, the composition of a book often involved the employment of a small army of scribes and secretaries, to whom the author dictated. Sometimes one copy was loaned or hired out for copying, and works existing in a single copy could be multiplied in this way, either by a single scribe or by a person dictating to one or more scribes. The problem of correcting mistakes was great, and it was not always carried out very well, so that errors are repeated again and again by copyists, who were often slaves, and had no personal interest in producing an accurate text. We can hardly be surprised that scholars who have had to edit the text of old books from the first century, whether they are gospels and epistles, or works by ordinary Greek and Latin authors, often have a hard task to determine the true reading of some passages.
from pages 119-120, Everyday Life in New Testament Times