Quote from polpolik:
My previous experience will attest to phantom orders being executed. I've had this happen multiple times. Orders I have clearly cancelled only to execute up to HOURS later. I've talked to them about it but nothing ever comes out of it. Fortunately for me, they're usually 1 lots and haven't been that huge of a hit on my account but I hear you and yeah, I'd sue the hell out of it. I won't let 52k go just like that.
edit (additional info)
and yeah, I clearly don't see any outstanding order but then out of nowhere, I'd hear the "ding" sound and then look to see an order filled, hours after I have cancelled it.
Sounds to me like more than one person has experienced this - and that makes it a bug. A bug in the software can be proven, and if a lawyer is the only way to get your money back, I'd do it.