Yes, in some cases it's possible to post without registering. We made that possible only in the Site Trouble/Feedback area so that potential members trying to join would have some way to contact us or get assistance with some registration problem.
Mostly though, when people have problems like that they use the "contact us" form found on the bottom bar of the forum. That sends an email to both Frederik and myself, and we take it from there.
Also "guest" may appear on some very old postings where the users account was deleted or merged into another one (in the case of multiple accounts from the same IP address). There have been instances where we were lawfully required to delete an account for reasons that are classified.