The contents of each file that is open does not show up. It is as if the window is not being painted.
When a document window stops painting it generally indicates that something unexpected (i.e an internal Zeus error) has gone wrong
On a few occasions I too have seen this happen, but unfortunately I was never able to easily replicate the problem at will. I think the problem is somehow timing related, which is why it is so difficult to replicate.
The next Zeus patch adds a few more internal locks and these will hopefully eliminate this timing error.
But if you find a way to easily replicate the bug please post the details