This sounds like a caching bug, but I'm not entirely sure where this would happen so I've got a couple questions:
a) when you say you're going to the discussion thread on Clearspace, is it the main discussions page (/clearspace/main-threads.jspa) or the discussions page for the community (/clearspace/community/foo/bar?view=discussions)?
b) Are there any errors in your log files?
a) I mean the discussions page for the community. (I got to the actual message via the link in the email notification: /clearspace/thread/thread#).
b) We don't see any email-specific errors on day in question. What kind of search string should we be looking for in the logs if this happens again?