You would end up paying $0.50 per share more for the stock than you intended, and you may have decided against buying the stock if you knew you were going to have to pay that much.
While a stock is unlikely to move that much between the time that you start placing the order and the time that the order is executed, smaller movements are common, and those may be frustrating if the stock moves against your position.
If you're price-sensitive about the stock or asset you're buying, you're better off using a limit order or a similar kind of order to ensure that you can control the sale or purchase price