When I first worked for the Bodleian, in the mid-1990s, I became aware of an issue that still causes trouble.
If, hypothetically, the Bodleian had acquired a manuscript on the third day of a five-day sale which began on 1 January, they recorded it as having been acquired on 3 January. Of course this is completely accurate, and represents a historical fact. A problem arises, however, because most people would cite the auction only by its start-date, in this case 1 January, which creates an apparent contradiction. It can then cause a reader some trouble, until s/he realises that the "1 January" catalogue and the "3 January" acquisition both refer to the same auction.
What I realised clearly for the first time is that most people conflate two very different and separate things: on one hand there is an event in the past, the auction, and on the other hand here is a bibliographical entity, the auction catalogue, that may not represent the historical event with complete accuracy.
There are numerous ways in which a catalogue can differ from the auction to which they relate. Items can be added or withdrawn from a sale after the catalogue is printed. In some cases a catalogue is printed and distributed, but the auction never takes place at all (see an example of this in this post). In other words, the bibliographical item and the historical event are not the same thing, and should not be treated as if they are.
Less dramatically, "saleroom notices" may be announced verbally and distributed as ephemeral pieces of paper at an auction to correct errors or add significant new information to the catalogue, stating, for example, that lot 1 is on paper not parchment; lot 2 is in Italian not Latin; lot 3 lacks a leaf and is thus not complete; the heraldry in lot 4 has now been identified; lot 5 has 322 leaves not 223 leaves; the estimate for lot 6 should read "£10,000-20,000" not "£1,000-2000", and so on.
Another example of the discrepancy between the auction and its printed record becomes obvious if we consider the hypothetical sale above, which began on 1 January: clearly the bibliographical item, the catalogue, must have been published in advance and so, strictly speaking, the auction and its catalogue belong to different years.
This preamble is really just meant to alert any readers who have never given the matter much thought, that it is always worth consulting a copy of an auction catalogue before they cite it, and that it should then not be treated as if it is necessarily a totally reliable record of the auction itself.
What started this train of thought is that the Gulbenkain collection includes a large (395×280mm) early 14th-century Parisian Bible, acquired at Sotheby's on 24 July 1924. Pursuing this in the Bibliography, I found that between the catalogues of the 1921 Yates Thompson sale and a 29 July 1924 catalogue naming R.H. Roberts, the catalogue cites another 1924 Sotheby's catalogue with no title, and no named vendor, just "[Sotheby's sale catalogue]":
Sotheby, Wilkinson & Hodge, Catalogue of a Selected Portion of the Valuable Library at Ecton Hall, Northants., Sold by Order of the Owner, Col. H. G. Sotheby, D.S.O., M.V.O. ... 24th of July, 1924, and Following Day
As the title-page states: