I get this from time to time on Safari on my Mac too, not sure I've seen it since upgrade to the latest version on Mountain Lion though. Seemed to happen when leaving a xenforo site open in a tab, viewing another tab and coming back.
Doesn't happen every time, so not sure how to make it repeatable otherwise I'd have logged a bug.
This is a known problem, it has been mentioned earlier and by others. To reproduce it, you have to use a Mac with Lion, latest Safari (although it happened on older versions of OS X and Safari too). Then open up a few tabs with different XenForo pages and after being in a thread for a while, switch between the tabs. The thing is, it doesn't always happen. So you have to be patient and do this for a while.
Not saying it can't be client side (Safari), it could be, but something in XF does not like Safari, since it's specific to XF. So at least the source of the problem has to be found and reported to Apple if not a XF bug.
I have never seen it anywhere outside a XF installation, but maybe XF is using some specific technology / code that is causing it which may be used on other specific sites I have not come across yet....
I'm running the latest Safari and I can reproduce it right now.