(FORTUNE Magazine) – In recent weeks dozens of tiny U.S. companies have suddenly begun trading on the Berlin Stock Exchange--unbeknownst, it appears, to the companies themselves. "We are completely dumbfounded to learn that a corporation's stock can be listed for trading on an international stock exchange without the prior knowledge or consent of the corporation itself," said Ray Grimm, CEO of Carlsbad, Calif., cosmetics seller FemOne, in a May press release. Like most of the newest Berlin additions, the company, which trades for about 42 cents per share, is listed in the U.S. on the Over-the-Counter Bulletin Board, an electronic trading system for penny stocks that has long been viewed as a magnet for stock manipulators.

Indeed, many companies are now claiming that the Berlin listings were instigated at the behest of so-called naked short-sellers. Regular short-sellers make money by selling borrowed shares, in hope of buying them back after the price has plunged; naked shorters simply skip the borrowing process altogether and sell the stock anyway. In April new securities rules aimed at cracking down on this practice were enacted in the U.S. Critics say the naked shorts have simply moved their operation to Berlin. The Berlin exchange denies any wrongdoing. Still, shorting in Germany applies downward pressure on shares here just as effectively. That's one more reason for investors to stay far away from the murky world of penny stocks. --Janice Revell