I have presented here several times some interesting cases of torn leaves from the library of the Monastery of Apa Shenoute (or the White Monastery), which can be reconstructed out of two or more fragments which are presently scattered around the world (see HERE, HERE, HERE, HERE, or HERE). In my opinion, these damaged manuscripts give us an important clue about the extinction of the Coptic library at the White Monastery.
The current theory, which is quite old and unsatisfactory, is that the library fell into decay when Coptic language started to be forgotten by the monks. According to this hypothesis, when Arabic became the lingua franca of the Copts, the old parchment codices were abandoned somewhere in a remote corner of the monastery where they gradually decayed. Beginning with the second half of the 18th century, the monks sold piecemeal the manuscripts to various European travelers, breaking them into pieces in order to obtain a higher price. In this way, the leaves of the manuscripts were scattered throughout the world.
However, I think that a multitude of fragments, which join perfectly and do not exhibit signs of a natural form of decay, show that the codices were destroyed systematically and deliberately by someone. In my opinion, the White Monastery parchment fragments actually bear clear signs of trauma and mutilation done by human hand. It is possible that, at a certain point in the Arabic period, suppressing the monastic libraries was considered to be a necessity in order to extinguish Christian culture in Egypt. I will develop this hypothesis, together with Prof. Tito Orlandi, in a paper which we shall deliver at the next Coptological congress which will take place next September in Rome.
Until then, I shall introduce here the illustrative case of two fragments from a codex which contained the Proverbs of Solomon in Coptic. The first fragment is currently kept in the British Library and was identified by Walter Ewing Crum in his catalogue of Coptic manuscripts in this collection. The inventory number of this fragment is Or. 3579A(27) (= Crum no. 39). The second fragment, identified here for the first time, is housed in the National Library in Paris as BnF, Copte, 1324, fol. 293.
Here is a Photoshop collage which shows the two fragments joined together. The damage pattern indicates that they were torn to pieces on purpose.
As can be seen in the picture above, the London fragment (the superior one) still bears the ancient pagination of the leaf: 13-14. The fragments contain the text of Proverbs 4:13-27. To the same codex belonged several other fragments. These include: Paris, BnF, Copte, 1315, fol. 119 (pages 9-10); London, British Library, Or. 3579A(27) + Paris, BnF, Copte, 1324, fol. 293 (pages 13-14); Paris, BnF, Copte, 1315, fol. 83 (pages 41-42) Paris, BnF, Copte 1293, fol. 129 (pages 49-50); Paris, BnF, Copte 1293, foll. 123-128 (pages 51-63); Paris, BnF, Copte 1293, fol. 130 (101-102); Cairo, French Institute, Copte 150-151 (103-106).
 W.E. Crum, Catalogue of the Coptic Manuscripts in the British Museum (London: British Museum, 1905) 12-13 (= no. 39).
(Thank you, Paula Tutty)