This one's pre-existing and might need to be fixed for stables as well.
- Add some tags to one or more forum posts (e.g. "cool")
- Go to the site admin and disable the tags functionality.
- Go back to the forum
- Search using the text box with the tags keyword (e.g. "tags:cool"), you'll see the forum posts with the tag.
Same thing when you're on accessing the forum as a guest and you try to search starred discussions using the "starredonly:on" keyword.