For years, hedge funds have made millions betting against businesses, causing CEOs to object, call for the regulation of short-selling, and cry foul. It took a group of internet commenters on the website Reddit to make the financiers pay the price.

In late January, random denizens of Reddit’s “Wall Street Bets” section bid up the price of video-game retailer GameStop (traded under the symbol GME) and movie theater owner AMC so high that the trading platform, Robinhood, froze purchases of both.