I have been a member of the Moodle forums for a few years, but every so often (it seems to be perhaps every few months or so, perhaps when I have not visited the forums for some time and want to ask a new question), when I try to post a message, I get the following error/warning:
"To post in a forum you first need to become a member of the community."
If I then follow the "Become member of this community" link, I can indeed then make a post, but I am mildly puzzled as to why this happens. (Only mildly puzzled, I add. If it's just one of those sort of unusual minor bugs, I can live with it…)