For those of you using TDA's Command Center 2.0 in Firefox be VERY VERY careful before you submit your order that your order was actually understood correctly.
When you choose a stock in your portfolio and hit the BUY or SELL buttons the order blank comes pre-populated with the current ask/sell price and 100 shares as default. There have been quite a few times where I had changed the price in the blank, but when I hit submit, the original price was the one used, which requires me to immediately click the "Do not place" button to try again.
This could be VERY costly if you are trying to rush and don't notice you are placing an order at a price other than you intended. A few cents on a $30 stock is not that big a deal, but a few tenths of a cent on a hundred thousand penny stocks could be a huge deal.
surfstocks: It's possible you didn't make the mistake at all. It very well could have been a system mistake.
Beware of Software, it's programmed by humans.