I've done it before!
I filled out the sell and submited on accident, didn't have sufficient funds, so I didn't think anything about it. So, I filled out the the buy (which is what I was going for)! Then when my order got executed, it immediately sold for the same price I bought for, effectively giving Gox about $7! It sucks I know! When in a hurry to get your order in before the price changes too much, it's an easy thing to do!
What they could do is, when you click "submit", a confirm order pops up! Easy!