Quote from wally_:
Gee... I wish I read that sooner. That's what happened to me when using Auto Trader. I meant to sell, but ended up buying. Noticed that the market was moving against me so to close my position I bought again (thought I sold) and ended up with 2 contracts. The PL was getting even more negative so I bought again in panic (I guess), ending up with 3 open contracts. Added three more in the same way, and finally sold six of them when I realized that the only way to finish this thing out is to do the opposite. Fortunately the last 3 contracts were bought and sold at the same price. The net price -$350 plus comissions. Now I will only be using hot-keys.... A painful lesson, but I guess one has to make one's share of stupid mistakes to learn the ropes.
I set my close position at market order too: Ctrl+C.