I read an assertion that when Catholic monasteries maintained scriptoria for copying of texts, most of the monks did not bother to learn how to read. Supposedly they copied each page as if drawing a duplicate of an image, and that's why so many errors built up over time without being caught.
This was posted as an unsourced tangent ("Most scholars believe" etc) on an old forum post, and I want to doubt it, but I don't know enough about monks or scribes to tell whether it is plausible.
Paul Saenger believes that in the twelfth century there was a transition from an oral method of manuscript copying to a visual mode, which allowed the monks to work in silence. As a consequence he contends that with this transition it is possible that some monks came to copy texts without comprehension.
He cites the late medieval authors Petrarch and Jean Gerson as examples of contemporaries who decried the state of these copyists. Both men described such people as "painters" rather than scribes since they merely sought to faithfully represent the appearance of the letters and arrangement without any real understanding of the language itself. Paul Saenger Paul Saenger. Space Between Words: The Origins of Silent Reading (Stanford: Stanford University Press, 1997), 252.
See also: Anna Zayaruznaya's discussion of manuscript copying in her study "In Defense of Green Lines, or the Notation of B-flat in Early Ambrosian Antiphoners" in Ambrosiana at Harvard: New Sources of Milanese Chant ed. Thomas Forrest Kelly and Matthew Mugmon (Cambridge: Harvard University Press, 2010), 54.
No. Medieval scripts were very specific and are completely illegible if you aren't trained to read them; many letters look alike and they also have very specific abbreviations, not to mention the intricacy of illuminated letters. Scribes probably wouldn't have been the most proficient readers in the monastery, but they were at least semi-literate. Errors in copying are bound to happen, and paleographers know how to classify them (for example, "eye-skip" occurs when a scribe omits a chunk of text between 2 instances of the same word). Paleographers also group these errors into "families," which tells them which manuscripts were circulated across time and place.
In addition, we see specific patterns of spelling and syntactical changes in medieval Latin, which reflects the pronunciation of certain words in the communities that wrote those texts. For example, caelum (sky) could be spelled as celum or coelum (this vowel change of ae-->oe is seen in several Latin words). The spelling changes are clear evidence that scribes were making conscious editorial changes to a text to comply with the Latin that was being taught and spoken in their home communities.
If you're interested in medieval paleography, I'd recommend Christopher de Hamel's books; he has several excellent ones to choose from. Another good one is Introduction to Manuscript Studies, by Ray Clemens and Timothy Graham. Dr. Graham was my professor during my undergrad, and he's one of the best paleographers in the world. He taught me nearly everything I know about reading medieval texts, and as you can see, he quite literally wrote the book on the subject.