Google “Untitled (1950)” into your laptop and you’ll be offered a handsome framed print of Mark Rothko’s “distinctive multiform painting” of that title, for £100 (including delivery) from John Lewis. Do not think of giving it to Pierre Lagrange, even as une plaisanterie. He’s unlikely to see the joke.

He’s already the owner of a work with that name, and he paid a pre-crash $17 million for it. He thought he was buying a work by Jackson Pollock. Unfortunately, it seems that some of the pigments in it had not been invented before Jackson stopped composing and began decomposing in 1956. As I said, Lagrange is not amused. It’s not that he can’t take a joke, or even that he can’t take the hit, but a fake’s a fake, if that’s what it is, and nobody likes being conned.

This Pollockish picture illustrates a curiosity of the art world.  Jim Grant quotes Aline B Saarinen: “If a fake is so expert that even after the most thorough and trustworthy examination its authenticity is still open to doubt, is it not as satisfactory a work of art as if it were unequivocally genuine?” The answer is no, because beyond some point it ceases to be art and becomes a trophy like a yacht, a means of keeping the score among the very rich, who are more likely to view it as an alternative asset class. The authenticity is the only thing that really matters. Without that, it is merely a curiosity.

On the face of it, the case against Untitled (1950) looks pretty ominous, but it’s far from proven, and promises hours of innocent fun for observers unless the spoilsports settle quietly out of court. At best, there will be a permanent questionmark over the painting. Its added notoriety value is unlikely to compensate for the loss of complete authenticity. As for Lagrange himself, he’s also lost quite a bit of value elsewhere. He was the L in GLG, the hedge fund that was sold to Man Group for $1.6 billion in 2010.

The timing of the sale demonstrated the sort of market savvy that had made GLG so valuable. Outside shareholders were offered cash, while he and his two co-founders took Man shares and promised not to sell them for three years. Bad move. The Man price, around 240p then, is barely above a pound today. Still, at least the shares he got were undisputedly genuine.