The difference of high and low of the day for DJI was almost 1000! For the S&P it was 100! This is a 9% and 8.5% drop for the DJI ad S&P respectively! Luckily, I was sound asleep when this was happening. I am sure that this type of drop will give everyone a heart attack.
Rolfe Winkler has the story. It could be some trader keying wrongly. However, there are a few wrong trades than just a single error.