I just handled a flag on a comment by a user with exactly 1 reputation, which was left on a question belonging to a completely different user. The user who left this comment has never gained a single upvote. The only votes their posts have received so far have been downvotes and close votes (the poor sap). So, as far as I can tell, this user has never gained the privilege to post comments during the lifetime of their account. It doesn't appear as an answer to the question that was "converted to a comment by" a moderator either.
The comment simply says "I need some help" followed by a link to one of their questions, which leads me to believe that the user tried posting an answer, which was then deemed "Trivial answer converted to comment" by the system and submitted as a comment automatically — which would explain why it doesn't appear as a deleted answer to the question.
Given that moderators have a policy to never convert answers to comments regardless of how useful they are as long as the owner does not have the privilege to post comments, why does the system happily oblige?