Unlike catalogues raisonnés for modern and contemporary artists, those for Old Masters are few and far between, and a significant number of the paintings are not autographed. “I’m standing here in front of three paintings, by [Francesco] Solimena, Guido Reni and [Gaetano] Gandolfi,” said George Wachter, the cochairman of Sotheby’s Old Master paintings worldwide, referring to lots offered in the firm’s Old Masters sale in New York this past January. “Not one of them is signed.” The Reni (est. $700–900,000) brought $959,400; the Solimena (est. $700–900,000) made $825,000; and the Gandolfi (est. $300–500,000) fetched $657,000.

Publication deadlines for auction catalogues or a seller’s impatience to part with a work before sufficient research can be done often results in cautious attributions, such as “from the workshop of” or “by a follower of” a name-brand artist or “in the style of” a certain school or century. “Auction houses have to be very careful about how they catalogue things, because they’re liable for a period of time, particularly if they catalogue something in error,” says Nicholas Hall, the international head of Old Master paintings at Christie’s. Sotheby’s Wachter adds: “There are times when we either pull something out of a sale because we can’t come to clarity on it or call it less than what it might be because we’re not sure.”

That was the case at Sotheby’s London last December with the page-size, gold-ground tempera on panel Madonna and Child Enthroned. Wachter and his colleagues believed it was probably by Giovanni di Paolo, based on the opinions of the scholars Laurence B. Kanter and Everett Fahy (like expert witnesses at trials, such advisers may be compensated for their evidence). But they catalogued it as Sienese school, early 15th century. “We couldn’t get a full agreement [on the di Paolo attribution] before the catalogue was sent to the printer,” Wachter says, “although over time it became clear that that’s what it was.” Estimated between £70,000 and £90,000 ($144–186,000), it sold for an impressive £412,500 ($840,000).

Wachter, a 30-plus-year veteran of the Old Masters market, recalls a historic blooper at Sotheby’s London in October 1995. In the sale of the former St. James dealer Hal O’Nians’s collection, a painting attributed to the 17th-century Italian history painter Pietro Testa was estimated at £10,000 to £15,000 ($16–24,000). The work, titled The Sack of Carthage, was purchased by Hazlitt’s John Morton Morris for £155,500 ($244,100), reportedly on the advice of the British scholar Denis Mahon, who had identified it as a very early Nicolas Poussin. Three years later, the picture—rightly attributed to the 17th-century French painter and renamed The Destruction and Sack of the Temple of Jerusalem—was purchased from the gallery for a reported $7.2 million. It is now a star in the collection of Jerusalem’s Israel Museum.

What about the original consignors of the “Pietro Testa”? Shortchanged sellers can’t expect much in the way of redress. In this case, though, Sotheby’s “had to pay off the owners, because we had not given them good service, since it was underattributed,” says Wachter. “We didn’t check the way we should have.”

In a similar case last year, the boutique Belgian auction firm Hôtel des Ventes Flagey sold a painting of a man snoozing on a chair in a café (est. €250–350; $340–480) for €4,600 ($6,600) to the local dealer Klaas Müller, who subsequently sold it to a Dutch collector for an unknown sum. The consignors, heirs of Count de Brouckhoven de Bergeyck, had been referred to the smaller firm by the Sotheby’s Brussels office, which had identified the work as an “anonymous 19th-century scène de cabaret.” Six months later it appeared at Sotheby’s Amsterdam, attributed to the 17th-century Flemish genre painter Adriaen Brouwer (est. €100–150,000; $144–217,000). It went for €360,000 ($520,000) to Old Master dealers Bruce Livie and Otto Naumann, who sold it this past March at TEFAF Maastricht, again doubling the price.