Correct me if I sound like the noob that I am, but ... You're right, because ... who's going to pay for the order if the exchange executes it?
I've actually done that before. I bought 1000 shares of something, and it ate up all my margin too. I sold immediately, but my broker didn't seem to care too much. They just said something about they would charge me interest for the remaining shares I didn't have money for.
Retail brokers allow 50% margin, which means that you can buy twice the amount of cash you have in your account. If you have $25K or over you'll get 4x intraday margin (i.e. 25K cash = 100k intraday buying power). Most likely your broker will not allow you to place an order for 250,000 shares unless you have the required buying power. Even if it does allow you and you get filled then you'll pay a hefty amount in margin interest. Either way you'll be in deep sh*t.