In 1998, Jeanne Marchig sold a portrait at Christie's in New York for $19,000 (plus Buyer's Premium). It was hardly the most remarkable sale that the auction house held that year, but perhaps what you would expect for a work which was 'German school, early 19th century' without further specifications.
However, the chalk, pen and ink work on vellum, known as A Young Girl in Profile in Renaissance Dress was the subject of interest for an Oxford of the History of Art, Martin Kemp. In fact, he devoted a book to arguing that the work is not German, nor 19th century, but an unknown work by Leonardo da Vinci.
As we reported, Kemp's ideas seemed to be confirmed not simply by artistic judgment, but by the trace of a fingerprint, which has been left level with the browline of the subject, and matches a fingerprint on another da Vinci work - one which he is likely to have created alone.
Carbon dating also seems to suggest that the painting is more likely to have been created in the 15th century than the 19th century.
On this news, the estimate for the worth of the painting shot up by a factor of over 5,000, with insurance put in place for $100m.
Now, Marchig is threatening to sue Christie's for failing to take proper care during its valuation. The claim will make reference to the current estimate for the painting.
Christie's for its part is not impressed, describing the claims as "without merit". Kemp's theory is by no means universally accepted, given that da Vinci is not known to have produced a work on vellum, and the fingerprint is unclear.
We will of course bring you any updates on this as they come. For now, the situation is a useful reminder that the vast differences in how valuable two collectibles are can be subtle, or even invisible, and that careful, expert assessment is crucial.