I was *under* the impression that once you put an order in 50BTC @ 10.5 that it was locked-in until sold.
It's called a limit-order. You limit the price to an exact value, and until the order is fully executed (e.g. one or many buyers buy 50BTC from you by 10.5) it's gonna remain in the system. At least that's how spot-trading on real exchanges work.
If there is much liquidity, it's possible for an exchange to execute market-orders, which would just fullfill as much existing buy orders as you specified. The price may be significanly worse for you in this case (obviously).