To help you trade in a smarter, more efficient manner, here are some important considerations. Although the Internet makes it seem as if you have a direct connection to the securities market, you don't. When such conditions occur, use common sense and added caution. By the time you get to the front of the line, the price of the stock could be very different. One way for investors to protect themselves from a rapid change in the price of a stock is to use a limit order rather than a market order.
Market orders are subject to immediate execution.
During market hours it is rarely possible to cancel a market order. But, the fact is it's canceled only when the market gets the cancellation. Changing an Order. When attempting to change an open order, be sure to enter it as a change order, not as a new order. If not, there will be no change in the original order and a second order possibly could be executed.