Consider yourself lucky you weren't typing a large catch order. Imagine someone wanting to type "Buy 8000 BTC if price falls to 8" and actually inputting "Sell 8000 BTC limit 8".
Where on MtGox do you type using syntax like you are using above ("Buy 8000 BTC if price falls to 8")? I only know the Trade Section where you have fields for the amount of BTC and the price. Or are you using their JSON interface from your own client?
I agree though that their GUI could improve a lot. It could also be more helpful to pre-calculate fields. Say you have $1000 to invest and want to buy BTC for not more than 14.0. You currently have to calculate yourself that you want to buy 71.44 BTC.
There could also be a warning if the price is *way off* the going price both when buying and selling