The thick, polished animal skin used as paper during the Medieval era wasn't very strong. It was really easy to rip during cleaning and preparation. But nobody could afford to toss away a page just because of some holes. So illustrators and scribes created art out of the flaws in their pages.
Medieval book historian Erik Kwakkel found these amazing examples of cleer.
Bearded face from a 12th century manuscript, a commentary to the Song of Songs
A Dragon in its cave, in a 9th century manuscript
From a 9th century book
(via Medieval Books)
A mid-14th century Magna Carta
(via Reading Revolutions)
Holes fixed with coloured threads, 12th-14th century
The contours of these patch repairs were discreetly hidden by illustrations
Silk broidery in a 14th century book, repaired by some nuns who purchased the book in 1417
The images are from Erik Kwakkel's awesome Tumblr blog, except when noted otherwise.