Fat Finger. I typed a futures price into a valuation spreadsheet that was out by 100 points, then proceeded to fire off about 2500 mispriced orders, half of which executed instantly. Down about a hundred grand in the space of 10 minutes, mostly due to IB autoliquidating me with market orders.
The feeling is kind of like having your limbs fall asleep, being kicked in the nuts, punched in the gut and getting the wind knocked out of you, all at the same time.