I hear that flash orders are likely to be banned on NASDAQ next year. Flash orders are submitted by firms that have co-located servers in the actual exchanges. It is known as 'flash messaging' and is a menace to most traders trying to place a trade only to find that the liquidity disappears as soon as they click the button, the trade is not filled and the prices move away such that you'd have to accept a worse price to get a trade filled immediately. The very liquid stocks don't suffer in the same way because there is usually genuine liquidity. The illiquid stocks suffer the most from flash orders, which appear to be there to spoof you.
Many times it will be somebody posting a market based on the relative value of another making it look like mindless tinkering. ...greg