Tonight, one of my players was goofing off with the drawing tools during some downtime, and he discovered that if he zoomed in to 150% it reset his camera, and the fog of war disappeared for him.
He was using Firefox at the time. If you need more details, let me know and I'll get them from him.
He said that refreshing the page corrected the problem, but he was able to reproduce it by zooming in again.
My other players were using Chrome, and were unable to reproduce the issue. (I tried as well.
I tried this myself using Firefox 11.0 and was unable to do it. Updated to FF 12.0 and still couldn't reproduce it.
Not sure what kind of stats you'd like me to try and get from him to try and figure this out.