We have heard enough about the recent flash crash of the market, but what about a particular stock? Does simply placing a big 'buy order' or 'sell order' at the market price cause it?
1) It would depend a little on what the day's trend of the stock/market is. To attempt to execute a "large" sell-order in a declining market would be more likely to cause a flash-crash. 2) To attempt to execute a "large" buy-order in a rising market can feed the rally but probably more slowly. When the order is filled, the stock would become vulnerable to decline without continued buying after a substantial upside move.
You need level II quotes (how many shares are available to trade at each price level). Then you spot a moment of low liquidity, and IMO a limit order (to buy above the quoted market price, or to short below the quoted price), is more effective than a market order.