Someone hacked the AP's twitter account and said something Really Bad happened, which apparently caused a brief, abrupt drop in stock markets, which then promptly recovered. So, if you got suckered during that brief period of time, you felt really, really bad (and potentially did something Stupid), very briefly, until you found out you'd been suckered, and then you felt great relief.
I think the real lesson here is that there is a community of stock traders that needs to develop a thicker skin: they need to be a bit more skeptical, in general, to avoid getting suckered, but more than that, they need to quit believing that events like this (real or not) will translate predictably into a certain outcome in the economy. _The Big Short_ has a particularly ridiculous example of a hedge fund which correctly predicted a coup in Thailand (not has hard to do as you might think) and thought that a coup would negatively impact the baht. Which honestly, is so fucking stupid I cannot get over it, every time I think of it. Coups either do nothing, or help the baht. The idea a coup in Thailand would harm the baht is just ... ridiculous.
But it turns out there are a lot of little rules of thumb like this that people _ought_ to know if they're going to be making these kinds of bets. But they don't.
Some of them will eventually learn.
ETA: Further thought (and a little research) suggests that perhaps no actual people were involved. This may have all been driven by algorithmic trading, computers reading the news and responding either by selling (possible but not likely) or no longer buying (more likely). If everyone takes a simultaneous pause on buying, anyone selling with some automation built in will tend to ratchet the price down and as the price drops you can trigger a lot of dumb stop orders. Not Good.