I don't know why this happen to me: I ordered to buy 7000 shares of a stock priced at 6.10 ($24000 cash plus almost full margin), but cancelled the order in a few minutes. After confirming the status of that order is "cancelled", I made another order of buying 8000 shares of the same stock priced at 5.90. This order was filled quickly. The price of that stock kept dropping after I bought it to 5.30 or lower. I thought I have 8000 shares in hand and maybe I can still take the loss . But the next day when my account was updated, it showed that I had 15000 shares and had a margin call around $20000!! I called my broker and they said it's Nasdaq's problem, as sometimes when canceling the trade, the order may already been filled, but the status may remain the same until the next day etc, and there is nothing they can do about it. Does anyone know whether their explanation is right and I have to take more than double the loss? If they are wrong, is there anything I can do about this? Your help will be much appreciated.