I did some futher investigations:
I downloaded Google Chrome Portable - and it is ok here...
I connected to a RDHost (Win2k8 r2 using ie9) - same problem. the first 10 secs are ok.. but after the first switch/slide, there it is.. I also used private mode and cleared all cache.
A better reproduce description:
1. Go to the 'test messages' forum
2. Select the 2nd tab of the notices and click on it immediatly.
3. wait until the slide switches to the next tab and wait 2 secs
4. click the 2nd tab and it will switch again after 1 sec.
It's like the IE9 would lose it's onmouseover event (or what ever event it is used here) after the click...
With Internet Explorer 8 I don't need to select a tab to see this behavior. The second notice ("Message Count") is only visible for 1 sec. If I go to another browser window, than go back, everything seems to be fine. I see both notices for about 3 secs. (Win 7x64)