The answer is in the thread title, ie bugs.
Any FBW aircraft is one very large IT project, it just happens to have wings and engines attached etc.
As one with a previous career in the IT industry I can tell you that it is an inescapable fact that ALL IT systems are riddled with bugs whether they may be logic errors or syntax errors etc.
Despite the most rigorous testing, any software driving any system whether it be an aircraft or a can opener will be carrying many many BUGS. The fact that we encounter them so rarely and they are of such insignificance is the only thing that surprises me.
It's not a Boeing vs Airbus thing it's a FBW thing.