How much is a picture of a cow staring at a picture of another cow worth? £1.36m, apparently. At least that's what Robert Wylde paid for Mark Tansey's 1981 work The Innocent Eye Test, which features just such a tableau, two years ago.

Yet it isn't the price that has caused a fuss, but rather the fact that the work turns out to be 31 per cent owned by New York's Metropolitan Museum of Art.

Now Wylde is suing New York's Gagosian Gallery, which sold him the work, claiming that they never told him of the Met's partial ownership of the piece.

The Gagosian says that it simply did not know that the Met had ownership rights over the painting. That defence in itself raises a rather awkward question. How is it possible for a gallery to sell a work for that price and not know absolutely everything about the piece? Did no one think to check before swiping the Amex card?

Still, the saga will probably end up adding some value to the work. It was intended by Tansey to be a satirical take on art critics. Now it will perhaps be seen as a satirical take on art buyers and sellers, too. And maybe the case will serve as a salutary warning to gallerists everywhere: attempt to milk your customers at your own peril.