If I place an order to buy at 3.00, for example, can the broker change my limit price to 2.99? This isn't a hypothetical, IB is doing it to me for some reason. And I'm missing fills because of it. Also, is a broker obligated to fill your order to all exchanges? For example, let's say that there are 10,000 shares offered at 3.00...5,000 are offered through INET, and the other 5,000 are offered through ARCA. I use a smart order to buy 10,000 shares at 3.00, and I get filled for 5,000 through INET. The other 5,000 shares offered through ARCA are still available, even though I still have a bid of 5,000 shares at 3.00. (Which for some reason will probably change to 2.99). Before I can resubmit my order, or go direct to ARCA, someone else has taken those 5,000 shares.