In the post "How do I pass an expanded optional argument using expl3/xparse?" there are two comments at the bottom by Bruno and Joseph. surprisingly (to me at least) Joseph's answer is logged one hour after Bruno's question. How did this happen?
It's not a time issue.
Bruno wrote a comment to the answer before, and it has been deleted. I guess by himself. The reason could be that Joseph has fixed the problem mentioned by Bruno, so the comment has become obsolete. It's good to remove obsolete comments, also Joseph's comment regarding that it's fixed could be deleted now. Obsolete comments can be flagged.
Interesting: I flagged Josephs comment as obsolete, then it has been automatically deleted, obviously because a moderator set this flag.