Browser: IE 10.0.9200.16384 (desktop mode, not modern)
OS: Windows 8 Enterprise
When: About 13:30 UTC on the 22nd of March 2013
In chat, the little blue chat flag notification popped up on my avatar. I clicked it a few times, but was never presented with the popup to actually preview the offending comment and vote on it. After a few seconds, the notification went away (likely because it was handled by others).
Chat flags are infrequent enough that I'm not waiting for another to see if I can repro before posting, so if it's a one-time client-side quirk, then my bad!