Quote from tomcole:
I'd insist they split it with you, and no commissions.
It is a MUTUAL error and most likely you'd win in a court. You'd argue the broker had every reason to know you were a mini-trader, didnt listen carefully and because his commiss is based upon transaction had a reason to enter a wrong order.
The exact wording on the tape plays a role as well - if it was done in a very hurried fashion as many brokers do, you'd argue they did not take reasonable care. Obviously a small mini-trader can not be expected to act logically under extreme duress as you were at the time.
If on the other hand, the broker repeats the order in a s-l-o-w fashion, gives every d-e-t-a-i-l and asks you to r-e-p-e-a-t the order in a slow fashion, you're porked.