Your market order will generally fail due to insufficient funds. This can happen if the price moves in the overnight hours and the overall value of the transaction exceeds your available funds. Let's say you put a market order for a $100 share of FB over the weekend and $100 is your current available funds. However, on Monday when the market opens, FB opens at $110. The current value of the trade (1 share x $110) exceeds the $100 you have in your account, so the order cannot be executed and will be rejected.
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