We cancelled an order in to buy 10k shares of a stock as it was nearing the close of the day and could not hold the position, and did not want to hold the position over night. The order had already disappeared from the screen, when suddenly heard a beap, indicating the order went through, and had to sell at a loss. Emailed IB and am waiting to see if they cooperate to arrange to bust the trade. I know Thinkorswim, Choicetrade, and a number of other brokers would do whatever they can to help a customer bust a wrongul trade.