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. For example, if over the weekend you put in a market order to buy 1 AAPL share (and $100 is your current available funds for new trades). However, on Monday when the market opens, AAPL 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.