Quote from capline:
the price I got was something like 106.95 when the pair was 105.88. I was immediately down 10k, wiping away a 6k profit.
If that's indeed the case, 105.88 to 106.95 is 1.01% difference, so your fill is >0.5% away from the market. You can ask for cancellation in IDEALPRO; not sure if this also applies to IDEAL.
Also, from what I know, if you send a market order to IDEALPRO, it will "look" for the best price and execute either in IDEALPRO or IDEAL. According to what seems to have happened to you, this does not hold for orders sent to IDEAL... maybe someone from IB can clarify this?
I also had 2 horrible fills recently, one of which is---in my opinion---questionable: http://www.elitetrader.com/vb/showthread.php?s=&threadid=136951
My trouble ticket is still under review after a week...