when you mouse over the top navigation icons (home, profile, friends, mail) it takes a split second for the hover icon to show. So it's blank for aquick second before it loads
I have a 10mb highspeed broadband connection so it's certainly not a slow connection!
The problem is in your css containers. The hover item should be loaded with the normal icon which is standard css coding practise.
I understand that once it's loaded it won't flicker again until I come back to the website another time. However this should not happen at all and I would consider it a bug.