But the $8 billion question is: are we talking about an obscure bug, a missed case in an otherwise perfectly sound valuation and risk management model, or is it actually a case of dodgy valuation and risk modelling? Which implies that many well-meaning clients are also seeing the wrong portfolio value, and trading with invalid margins?
Again, I don't have the details so I don't want to speculate too much, but apparently they've had similar "bugs", so it's possible that their entire valuation and risk model is dodgy. It has happened to more reputable organizations.
If their stuff was generally sound except for this, they would have shut down margin trading until the bug is fixed.
I don't think small bugs in high quality shops would fall under this argument.
For brokers in particular, they are highly regulated and I can't imagine them not ending up with a nasty investigation+large fine from regulators over this.