The stock market saw new highs on Wednesday, even though gains for most major benchmarks were extremely small. Investors are trying to balance their enthusiasm about the U.S. economy and potential future benefits for corporations whose stock they own against the possibility that the stock market's valuation is getting overextended. Oil prices also fell below the $50-per-barrel level, causing some nervousness that the energy market's recent recovery could reverse itself. Some individual companies had to deal with substantial downward pressure on their share prices, and Shopify (NYSE:SHOP), Momenta Pharmaceuticals (NASDAQ:MNTA), and MBIA (NYSE:MBI) were among the worst performers on the day. Below, we'll look more closely at these stocks to tell you why they did so poorly.

Shopify leaves a sour taste in one analyst's mouth

Shares of Shopify dropped more than 11.5% after famed short-selling analyst company Citron Research targeted the website and e-commerce specialist. Citron analyst Andrew Left suggested that Spotify's share price would be more appropriate at $60 per share, or roughly half where it started the day. Citron alleged that Shopify's business model relies on promotion of get-rich-quick themes, but Shopify has focused investor attention on the quality of its e-commerce platform. The stock has undoubtedly climbed dramatically, having doubled in price since the beginning of 2017, but many bullish investors still see plenty of upward potential for Shopify going forward.

Shopify logo of green bag with letter S on it, followed by name in lowercase letters.

Image source: Shopify.

Momenta takes a hit

Momenta Pharmaceuticals stock plunged 14% in the wake of a competitor's unexpected victory in producing a generic equivalent to a key multiple sclerosis drug. The U.S. Food and Drug Administration surprised investors by approving a generic alternative to Copaxone, opening the door to a potential fight between the generic producer and the company that developed the original drug. The news is bad for Momenta because it's part of a partnership that produces a biosimilar version of the drug, and investors believe that physicians are likely to prescribe the new generic alternative rather than Momenta's biosimilar going forward. Moreover, even if Momenta gets another version of its offering approved, it will still potentially have to wait in line behind the drug the FDA approved today, and that's far from ideal for shareholders.

MBIA watches Puerto Rico

Finally, shares of MBIA fell more than 8%. The bond insurance specialist reacted negatively to comments from President Donald Trump that said that holders of Puerto Rico-issued municipal bonds should expect to receive little or nothing from the U.S. territory in the wake of Hurricane Maria. MBIA worked hard to get back on track following the financial crisis, but to the extent that it has guaranteed repayment of Puerto Rican obligations, any move from the possession to default could lead to potential liability for MBIA. That's one reason why MBIA has participated in legal disputes over Puerto Rico's debt, but the involvement of the U.S. president is a new wrinkle for the financial institution.

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