I've had this issue for a while and am surprised that nobody has asked about it here.
Sometimes, while waiting for a new question on the SO frontpage, I notice that my rep changes a little. Curious as to which post earned me that delta-rep, I mouseover my username to see the latest rep-change. When I do this, I don't see any new rep changes. Refreshing the browser clears this problem and the dropdown is now populated with the latest rep changes.
Is this a bug in SO, or a browser specific thing or… ?
I'm using Mac OSX 10.7.5 and viewing SO on a dedicated fluid.app app.