I expected this to happen so trained myself to always keep in mind whether typing in the upper or lower box.

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".

This is a major interface flaw of Mt. Gox. I consider it so dangerous that I warn people I execute trades for that the mistake might happen to me one day. Even though I know of it, when speedily inputting orders on a small screen, the mistake is just very likely. In the end, Mt. Gox is ghetto, we all know that. Be happy as long as they figure out why their trading engine displays impossible spikes.