I stumbled about this question when it was eight hours old.
A new user (not the question author) commented under the question. The reputation score of the new user is "one". Looking immediately at the user profile I see an (unregistered) beside the user name, also the user has not posted any questions or answers yet, so it is unlikely that the user had at least 50 reputation points when he posted the comment.
How can that be possible?
If you are referring to the 1-rep user Donald Duck's 'comment' on that question, that was initially made as an answer. Moderators such as myself reserve the power to convert non-answer answers to comments (automagically) when it's still worth keeping as a comment, and as was stated by another answer here, we sometimes use this when there's a "not an answer" flag on a post. (However this is one I found in the review queues and not via flags, - and yes, us moderators DO sometimes sift through the review queues to help things along).
We don't always do this action, though, for "not an answer" posts, but in this case I did because it was more asking questions to the original question poster than posting a "non answer" that served no useful purpose.
(Note that at the 10k rep level you'd see the deleted post as well and seen it was converted to comment by moderators.)
Moderators can convert an answer to a comment. This usually happens when the answer is flagged as "not an answer", the moderator agrees, but does not want to delete the post entirely.
