Ah, this is so great. The Walters Art Museum in Baltimore maintains a wonderful digital resource, The Digital Walters, where the public can access digitized manuscripts from the museum's collection. When I was in grad school, I studied in the Walters' collection, which meant I got to hold a 600-year-old book in my hands and turn its leaves.
I was pointed toward the image above by Ellen Lupton, in her book Thinking with Type. The page is from a book of hours made in England at the end of the 13th century. In the days of hand-copied codices, text was not set in an authorized version as it is in a modern printed book. Text was subject to errors made by scribes, errors that might be reproduced in subsequent copies until the mistakes became codified. (Scribes also made their own revisions, redactions, and additions to manuscripts, but that's another topic.)
In this case, the scribe realized there was an error on the page, and corrected it by lettering the proper sentence at the bottom. The monk clambering up the side of the page points to where the revision fits in the text.
All publications should adapt this method of correcting errors immediately.