Is this a broker problem or exchange? I use fidelity. I put in an order and I don't like the price action. So I go to cancel it and the option to cancel is disabled. I wait anywhere from 20 seconds to 10 minutes before that option is available. Meanwhile I end up buying/selling something I didn't want because I couldn't cancel it. (this was not first 10 minutes of the day). I have an order filled and set a sell limit shortly after because things went in my favor. I missed the fill, but again it takes seconds or a minute for fidelity to recognize my placed order before I have the option to cancel or replace it. Is it that the exchange hasn't recognized my original order, so it is not available to cancel yet? And then it gets executed? Other notes.. I usually use ARCA, and trade in 500 share sizes. I don't place orders the first 15 minutes after open. I use limit orders for everything. Any useful advice appreciated.