In a dingy corner of the internet is a message board, soaked in profanity, bro-speak, and greed, where posters with handles such as OverthrowYourMasters and yolo_tron campaign for their favorite stocks, putting up screenshots from their online brokerage accounts of their moonshot victories—or showing off their massive losses like badges of honor. Some of them think they’ve found the key to fast wins on the stock market. Wall Street doubts they’re right, but it’s getting nervous about what it sees there.