In what could rank as the grandest auction house underestimate of all time, a Chinese vase that was expected to sell at a Sotheby's on Tuesday for as little as $800 skyrocketed to an astronomical $18,002,500. Dated in the catalogue as "probably Republican Period," or from the early 20th century, the beautiful famille-rose porcelain vase prompted an explosion of bidding from experts who were convinced that the auction house had erred in their appraisal, and that the piece in fact hailed from the imperial-era Qianlong dynasty of the 18th century. Last year another vase believed to be of that vaunted provenance became the most expensive Chinese artwork to sell at auction when it similarly shocked observers by selling for $85.9 million at the tiny English Bainbridges company.

Gilt and embellished with exotic birds against a background of peonies and magnolias, and bearing gold-painted scepter handles, the vase was offered as part of "Informing the Eye of the Collector: Chinese Ceramics and Works of Art from J.T. Tai & Co," a marathon nine-hour single-owner sale of more than 300 lots that made a total of $36.3 million. Remarkably, the seemingly misdated vase accounted for half of that tally.

Apparently, Sotheby's had some indication the vase might be of an earlier vintage than the 1911-1945 Republican Period of revolutionary Chinese leader Sun Yat-sen, since the catalogue stated in part that "a group of vases each bearing six-character Qianlong seal marks and of the period, show decorative elements that are related to the unusual present example." In 20th-century examples, those markings were typically copied in the interior and on the underside of the vase in a way that meticulously mimed the earlier and greater imperial style exhibited in the one Sotheby's sold.

Sotheby's declined comment about the two-century-plus gap in dating apart from what the press release stated, that "the estimate reflected this dating" and that "there was a healthy debate surrounding the age of the piece, with a number of collectors clearly feeling it was significantly earlier." At least seven bidders chased the Qianlong trophy, which ultimately sold to an anonymous telephone bidder.

Ironically, the unrivaled connoisseurship attributed to the late seller, Tai Jun Tsei, aka J.T. Tai, who opened a famed New York gallery in 1950 after escaping Communist China, may have been undermined by the vase's stellar performance, considering that the "probably Republican Period" label now appears to be a magnificent blooper. After all, there is nothing unusual about the price, given a correct attribution — in addition to the spectacular Bainbridges vase, another piece from the period fetched $1.38 million at Freeman's auction house in Philadelphia last week.

The J.T. Tai sale also had another whopping misdate, or so it would seem, as another famille-rose vase identified as Republican Period shot to $1,314,000 against an estimate of $6,000-8,000. The piece went under the hammer immediately preceding the $18 million lot. It is understood that a portion of the auction's proceeds will benefit the late dealer's foundation, though Sotheby's declined to comment about the foundation's involvement.

But despite the dating glitches or just plain old marketing pitches in the New York sale, the name and aura of J.T. Tai is still golden. Last October at Sotheby's Hong Kong sale, for example, another Tai lot, a famille-rose double-gourd vase with yellow ground from the Qianlong era, bearing the same six-character imperial seal mark, sold to Hong Kong collector Alice Cheng for HK$253 million ($32.4 million.) Even with the correct dating, it was more than five times the high estimate.

"Nothing surprises me about the Chinese art market today," said London-based Chinese art dealer Ben Jannsens, reached at his stand at TEFAF in Maastricht after the extraordinary sale at Sotheby's. "My hunch is, it would be 18th century."