Marcus Shea:

Here’s the most likely scenario as to what happened here:

Somebody had a decent short position. Say they had about 65 shares (the volume bars indicate that this was likely a &lt- 100 share transaction, ie, &lt- $5 worth of commissions for Intrade, so mentioning commissions / greed as a motivator is pure ignorance). They wanted to put up a buy of 65 shares at say 5%, so that if the price ever dips that low, they can close their short position and wind up with a nice profit. And then, big mistake, they hit sell instead of buy. The market plummets. Mystery solved. It&#8217-s called human error.