I guess that occurs when you follow some misformatted link, e.g. http://perlmonks.com/?node_id=1024047 when logged in thru perlmonks.org, instead of Re: Once again ... is it time to get rid of the Anonymous Monk?
(or probably http://perlmonks.org/?node_id=1024047 when logged in thru perlmonks.com
Possibly not relevant if you checked "remember me" on all relevant domains
Update: and not "while posting", but rather "posting and later notice that I wasn't signed in since several pages". From user's perspective there's no difference :-)
Update 2: I want a "preview"-button when updating a post...