When Richard Klank bid $21,000 for an antique dower chest at an auction in December, he doubted it was authentic.

The pine chest, thought to be Pennsylvania-made, was painted with unicorns and flowers and dated 1788. Its value was estimated at $8,000 to $12,000 by Weschler's, a reputable Washington auction house commissioned to sell it by an elderly Richmond couple.

Promoted in national antique magazines before the sale, the chest reportedly had caught the eye of actor Bill Cosby, an avid collector whom Klank represents occasionally at auctions.

"It's said there are more fakes than not in the area of Pennsylvania Dutch-painted furniture and drawings," says Klank, a University of Maryland art professor whose usual expertise is Shaker furniture. "And I was unsure."

When the gavel struck, he had bought it anyway.

The next day, another expert confirmed his fears: The chest had most likely been repainted in the 1920s or '30s, diminishing its value. Klank refused to pick up or pay for the chest.

The dispute is now "in litigation," says William Weschler, one of the owners of the 96-year-old auction house whose policy on furniture is "what you see is what you get." While the item was considered "somewhat original," he says, the estimated value -- a sixth the retail price of similar authentic chests -- should have raised some eyebrows.

"We don't want to sell forgeries and fakes . . . " says Weschler, adding that his staff tries to weed out the ersatz pieces from volumes of antiques he handles on commission. "If we had known there was a problem, we would've posted an errata and made an announcement at the time of sale. As it is, all we have is a headache . . ."

Tales of headaches and heartaches among collectors of fine art, antiques and collectibles are becoming more common today.

A month ago, seven people were charged in New York with selling (for $3,000 each) hundreds of phony "signed" lithographs by Spanish surrealist Salvador Dali -- part of a $625 million worldwide bogus print racket.

A sour note accompanied last April's sale in London of a Stradivarius violin for more than $347,000: The vast majority of Strads bearing the label "Antonius Stradivarius, Cremonensis, faciebat Anno 1727," tucked away in closets throughout Europe and America, are cheap imitations.

A Massachusetts decoy salesman wrote in a letter appearing in Maine Antique Digest's March issue that while browsing at New York's Winter Antique Show, he stumbled on a familiar decoy -- "a reproduction preening goose that I have sold hundreds of (for $16). It was sitting in a dealer's booth (faked up) for $2,800."

With dramatic inflation of public interest -- and prices -- in the past 10 years, collecting has been riddled with forgeries, fakes and misrepresentation, leading experts and amateurs alike to voice this caveat: Sooner or later, every collector gets duped.

"You can expect to be taken at some point," stresses Charles F. Hummel, deputy director for collections at the Henry Francis du Pont Winterthur (Del.) Museum, the world's largest and most comprehensive collection of American decorative arts.

Hummel, who entitles his occasional lectures on the problem "Too True to Be Good," says the increase in collecting "mistakes" has been fueled by a classic rule of economics -- supply and demand.

"Certainly we've seen a rise of prices in the art market, and not only in paintings but also in decorative art objects and collectibles," he says, a few weeks after Christie's, the British auction house, sold the first million-dollar piece of American furniture -- a Philadelphia pie-crust, tilt-top tea table.

"The demand is great and has been growing steadily as people hunger to have good looking art objects as part of their lives. And when the demand is great and the supply is small, you are always going to have individuals ready to make up that gap."

Jim Cook, who has accumulated one of the foremost U.S. collections of shorebird and duck decoys, agrees that "lucrative finances" have crowded most areas of collecting. son investing in any collectible area is attracted to it because it is a good hobby and great and stimulating -- but also because it has probably been tremendously profitable," says Cook, founder of Investment Rarities Inc., a Minneapolis-based company that tops national sales in U.S. and rare coins.

From fine art to comic books, what was cheap 15 years ago is now "dear and expensive," says Cook, unabashed about his enthusiasm for collectibles whose price tags defy rationality, such as fishing plugs that have bobbed as high recently as $400 each.

"In duck decoys you see the same thing," says Cook. "You used to be able to buy a wonderful duck decoy for $25. That same decoy now could cost you $30,000. Two recently sold for $96,000, and this will probably be the first year when a duck decoy will sell for $100,000."

Cook blames the woes of the market as much on the avarice of unethical dealers as on con artists.

He recalls once as a young collector finding an 1837 U.S. Army flintlock pistol in a shop. "The dealer told me he'd gotten it from the grandson of a general at the armory John Brown had raided, or some fabulous story like that." Cook offered $2,500, but the dealer stood firm at $3,000 -- too good a piece, he said. Months later, an expert told Cook the gun's barrel had been redone, making it worth about $300.

"Unfortunately, the majority of dealers of collectibles will take advantage of the customer," says Cook.

While experts and collectors generally agree greed and misrepresentation are widespread, most resist a paranoia that finds it under every Windsor chair, weathervane or quilt. Hummel stresses that few objects start as fakes.

"Most begin as good quality reproductions made for legitimate purposes, perhaps in the late 19th or even 20th century, but cross over the line at some point and somebody accepts them as real." Once a piece is exhibited or labeled authentic in a catalogue, it acquires credentials that may become difficult to denounce.

Hummel mentions a pair of Chippendale period card tables at the Winterthur Museum that proved not to be two of a kind. "One was beautifully crafted, but not made by the same hand as the genuine table. It was not made 'in-period' . . . but it came to be regarded as antique."

Samuel Pennington, editor of Maine Antique Digest, a monthly tabloid published in Waldoboro, Maine, agrees unintended deceit is a big problem. "A lot of this stuff was probably quite innocent. Fifty years ago, you've got an old chest and the paint is getting bad, so you repainted it. There wasn't this grand passion for authenticity back then." But there's plenty of evidence of "deliberate doctoring," he adds.

One longtime collector thought she had a fine 18th-century Virginia spoon rack, only to find out it was made in Portugal, says John C. Newcomer, a dealer based in Keedysville, Md., whose expertise is folk art, country American furniture and Americana.

Newcomer rattles off popular items prone to tampering -- metalwork, touchmarks added to pewter, weathervanes, game boards, pie crust tilt-top tables fashioned from ordinary tilt-tops, silver flatware, pottery and porcelains, painted furniture, even carousel animals, always heavily painted and rarely handmade, which are typically confused with poor imitations mass-produced in Mexico. He estimates that of the collectibles and antiques for sale, "at least 20 to 25 percent isn't right -- it's forged, copied or misrepresented.

"The big thing now is crib quilts," says Newcomer, who offers clients a buy-back guarantee if a purchase isn't as he represents it. "They take a wonderful center out from a mediocre applique quilt and it brings much more money -- maybe $750 as a crib quilt instead of $300 or $400."

He tells of "an absolutely wonderful" wooden serpent weathervane, represented by a New York gallery as being made in New England in the 19th century. "A farmer had made it in the 1970s. When it came off the roof in a storm, he propped it against his barn. It disappeared -- until he spotted it years later in an antique publication. He'd put his name and the date on it in brass tacks, and the dealer who had it could see where the tacks had been removed. It had been resold two or three times as an antique."

Why are collectors and experts alike repeatedly "taken" by innocent or intended deception?

Weschler recalls in a recent auction, "We had a watercolor of two Indians signed A.J. Miller. We didn't think it was right, so we estimated the value at $200 to $300. Someone still paid $3,700 for it."

What sells fakes and problem pieces is people's desperation "to want something to be right," says Hummel. "In the trade, auction houses, dealers, museums officials -- they all want an object to be what they think it is. It is easy to rationalize things that are seen in an object -- this can be accounted for as wear, that can be accounted for as a craftsman's mistake.

"It ranges from innocent passion to not-so-innocent greed -- and every collector has to guard himself." CAPTION: Picture 1, Painted Pennsylvania chest, circa 1788. Experts say painting on front was probably done in the 1930s, making it worth one tenth it's actual value. WESCHLER & SON INC.; Picture 2, $40,000 Canada goose decoy, circa 1934, carved by Shang Wheeler. TOM STANLEY