It happened to me that my IOC order was sent, confirmed, then pending and then after a minute rejected. Several times.
This is an option for the time in force (TIF) where you can choose good until cancel (GTC), DAY or Immediate Or Cancel (IOC). You send a market order with IOC, and the behavior will be that the route for SOES or SuperSOES will seem to act as an ECN because the market makers will have to reply immediately. Many brokers support this option, seen it at AB Watley, Point Direx, etc (but unfortunately IB don't support it for the moment :-( ...