The above statement, by Henry Bradshaw, could be the official Creed of the provenance researcher.
When discussing cataloguing, I sometimes tell people that I think that the single most important thing a cataloguer can tell their reader about a manuscript is its collation. (This is partly, but not exclusively, because the physical structure of leaves and bifolia is one of the few features of a manuscript that usually cannot be conveyed by photography). People who study manuscripts -- and early printed books -- seem to fall into two camps: on one hand there are those who see collation as a mechanical task, like measuring the leaves and counting the number of lines, and on the other hand are those who really understand how much a book's physical structure can tell them about its manufacture, and know how to use this powerful tool to inform their work. I won't go into details here; I mention it only as background to explain why Henry Bradshaw is one of my bibliographical heroes -- on a par with M.R. James and Neil Ker.
Last summer Maria Saffiotti Dale, Curator at the Chazen Museum of Art, sent me just the sort of provenance puzzle I enjoy. It concerned a 15th-century Italian cutting, shown above, described on the Chazen website as an "Initial 'D' from an Antiphonary from Como Cathedral with a Temple". 
On the back is an inscription which she could not entirely read: "I can make out all the inscriptions EXCEPT the initials(?) immediately preceding the date 1856 and what follows it. Any ideas?"
The Schoenberg Institute at the Kislak Center at the University of Pennsylvania is the only institution, as far as I'm aware, that regularly introduces its medieval manuscripts by way of YouTube videos; there are now more than 70 (including non-western MSS). I only recently became aware that one of these videos concerns not a codex, but two collages of cuttings, perhaps assembled in the 19th century, one of which is shown above.
This caught my attention because I had also recently been looking at other fragments from the same manuscript.
Many readers of this blog will have known, or at least known of, Bernard ("Barney") Rosenthal, who died at the beginning of the year. There are online obituaries by John Schilman here and by Ian Jackson here (also here).
Trawling images on my hard drive this week for something completely different I came across another cutting, apparently unpublished, apparently from the same Gradual as the ones in a recent post, now at the Detroit Institute of Arts:
After Otto Ege's early death in 1951, his widow Louise inherited his estate, of which much of the value was his collection of medieval manuscript leaves. At the time of his death he had apparently done all the groundwork to prepare the portfolios of "Fifty Original Leaves", but did not live long enough to start marketing and selling them. In a previous post I looked at an example of Louise's attempts to market portfolios after his death.
It is usually assumed or implied that she sold leaves from books that he had already broken up, and that her role after his death was simply to try to recover the money that he had invested in the collection, but I have recently identified an example of a book that she apparently bought and broke herself.