What is freak trade in stock markets?

The 50-share Nifty hit its lower circuit filter Friday as spot prices dropped over 900 points to a low of 4,888. The sharp fall was probably on account of a freak trade - an erroneous punching order by a large trader. The sudden crash affected the BSE Sensex too, which dropped nearly 300 points to slip below the 19,000 mark. However, the trade was not executed on the BSE, so it was not affected as badly as the NSE, where trade had ...