The OP was asking about market orders. On the day when ES opened limit down, many stocks and ETFs opened WAY BELOW their prior close, despite any adverse news of that particular stock (or ETF). At first it was thought that the trades would be canceled, but they weren't. So anyone who sold "at market" or who had a market stop may have been filled at an unfortunate price (depending on whether you were a buyer or seller), where it was difficult to find price discovery. Many "odd lots" got filled (i.e. mom and pop orders for fractional shares). Also, those who bought "at market" may have paid the ask on a ridiculously wide spread. (If you saw the bid/ask on SPY options, the spread was INSANE).
Hence, those who either BOUGHT at market or SOLD at market may not have been filled at the optimum price, however all of the news claim that "price discovery" was met, which of course is subject to interpretation.
This was a rare instance where the NYSE invoked a seldom used rule called Rule 48. You can read about it here and on the net.
http://www.businessinsider.com/nyse-rival-says-handling-of-market-chaos-was-absurd-2015-9