In fast-moving markets stock prices can change very rapidly, particularly if you get involved in 'hot' stocks such as IPOs. As almost by definition a fast-moving market has a lot of trader activity, you can also find that the response time when you're placing trades online suffers, and the price has moved dramatically before your order can be filled.
If you suspect that the stock will change price rapidly, the first thing you should do is ignore market orders, and place a limit order which will control the price you pay for the stock. The buy limit order will set a maximum that you are prepared to pay for stock and if it cannot be bought for that price by the time the order comes to execution, then the order will not be filled. Of course that means you do not get into the trade and make any subsequent profit, but that is where you need to exercise skill in determining the level you are prepared to pay.
For instance, if there is a hot IPO the initial price may be $10 a share, and it will rise rapidly to $40 a share because of the irrational enthusiasm of the buyers, but by the end of the day it will sink back to $20 per share. You need to form your own opinion of where the price will be going, but obviously a market order that is delayed may be filled above $20, and you would have a loss by the end of the day. If instead you issued a buy limit order with a limit of, say, $18, your order would either be filled and you would make at least a slight profit (more if the price did not rise so quickly), or you would be kept out of the trade and hence not exposed to the loss.
You must bear in mind that the price you see on the screen may not be the price that is active in the market, and you may need to make due allowance so that you have a chance of getting in the trade. There can be various choke points in the online trading system, whether caused by your Internet provider, the inadequate equipment of an online broker, or generally heavy Internet traffic possibly particularly associated with the IPO. In a fast-moving market you may not have much time to react to an adverse move, so you should be sure to set your stop loss orders as quickly as possible.
Finally, it's always a good idea but especially so if you have a fast-moving market to have an alternate way of placing or cancelling trades. These methods may include using a touch tone telephone, faxing the order, or even talking to your broker on the phone. While some of these will cost more than an automated online trade, if you find yourself in an urgent situation because of the price fluctuations, you must make sure that you know your options.