I recently tried to add two comments in rapid succession on SU and I got this error message pop-up window:
As you can see, it’s unreadable, because the “Chatting” screen element is in front of it.
This seems to happen when IE8 is running in compatibility mode, at least that's how I managed to reproduce the issue.
Solution: check to see if compatibility mode is active, and if it is, disable it. There's two ways to do that:
if you see the compatibility mode icon next to the address bar, click on it:
alternatively, hit F12 to open Developer Tools, then select Browser Mode → Internet Explorer 8