I had a terrible experience with IB once. I daytraded the DAX (german index) future and the front end told me my position was closed out by their system. I could still see the contract on my position window though and still had the margin requirements. I called support, and they took 48 HOURS to tell me if I had a position or not. Because I didn't know if I had it or not, I could not even hedge!! It was terrifying..
Funny thing is, after 4 weeks of examinations on their part (and even without me threatening them or something alike), one very kind guy at IB that kept working on it finally had them pay me back the losses the whole story resulted in (something like 1500$). As soon I had gotten through the helpdesk and was in contact with him, things were on their way.
So, bottom line: They're cheap, and yes, they CAN screw up badly. But if you manage to get through to the competent guys, they are even trying to get things straightened out.
May something like this never happen to you!
Wonko