I couldn't reproduce this on 4.0.11. Might have been a core bug since fixed?. I think I have a fix for this now, based on Kevin's guidance. Having trouble with dev mode on the latest version though, so I'm afraid I can't test it properly or build it for release just yet. Okay. It's kind of complicated, but I think it's working as intended. The 'allowed prefixes' and 'defined tags' inputs on forums are one and the same internally. I split them apart on the forum form to try making it easier to understand. If no tags/prefixes are defined explicitly on a forum, it will automatically inherit the global predefined tags (if any). And because tags and prefixes are both stored there, if any of those global predefined tags are set up as formatted prefixes, they will be treated as prefixes. I would hazard a guess that two of those four global tags have prefixes associated with them, and the other two do not. What does that mean for you? You can fix it one of two ways. One, remove the global predefined tags and set them all per content container as needed. Or two, go into those affected forums and either define more proper tags, or change the tag mode there to remove the tag or prefix inputs for that forum. Okay, thanks. That should be fixed for the next update.