wily, it is an ISLD problem.
Let me see if I can 'splain it, and anyone feel free to correct me if I am wrong. Using the example of a stock that is taking a hard shake, and appears to be bottoming at 1/8 x 1/4.
You bid 3/16 for 1000 shares and someone kindly hits you for 100 shares. ISLD does not just change your displayed size from 10 to 9! Instead it pulls the 10, then re-displays a 9.
If, during that split second while ISLD is refreshing your order, some MM or another ecn drops the ask to 3/16, the balance of your order will be rejected for locking the market.
Now if someone else on ISLD (or a MM or another ecn) had joined you at the 3/16 bid, then, when your 900 share buy got refreshed by ISLD, you would be back at the top of the ISLD queue.
Worse yet, if you have "Sound Events" set up in RealTick, you will get a "Order Partialed" sound for the partial fill, and a "Order Filled" sound when the balance is rejected, so keep an eye on your position minder.
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