When Henry Francis du Pont was assembling the antique collection that would become the Winterthur (Del.) Museum, he often demanded of dealers, "You've shown me your treasures, now show me your mistakes." Du Pont insisted there was as much to learn from defeats as from victories.

Here are some classic mistakes:

* Charles F. Hummel of the Winterthur Museum: "Early on in my career, I collected brass sconces. I bought a pair that I thought were legitimate, late-17th century English. A year or so later , I discovered they were being produced in Great Britain in mass quantities, basically as souvenirs and were worth a quarter of what I paid . . . brass has been a big problem because so many are cast from old examples."

* Samuel Pennington, editor of Maine Antique Guide: "I bought a Queen Anne dropleaf table. Sat through four hours of the auction to get it and paid $600. I was willing to pay $1,000. When I took it outside in the sunlight, the top boards were walnut made from an old Victorian table. The legs didn't look any too kosher either -- probably built in the '20s. I didn't do my homework. It happens to everybody. The guy who says he hasn't bought a fake has never stuck his neck out -- or he's lying."

* John C. Newcomer, dealer of folk art and Americana: "In 1975, a miniature decorated chest, Pennsylvania-made, circa 1850. It turned out to have been made a few years earlier as a joy box and later was artificially aged with heat and chemicals. I paid $950."

* Jim Cook, founder of Invest Rarities Inc.: "I bought a painting of geese by Jacques in 1979 for $5,000. He's a deceased artist who is shown in the Natural History Museum in New York. It should've been worth $10,000. I put a black light on it . . . in a dark room and that'll show cracks, repairs, repaint on any material. The painting had been damaged and somebody just touched it up along the top. So that painting was worth probably $500 instead."