From MDL-65129:  

This one's pre-existing and might need to be fixed for stables as well.

1. Add some tags to one or more forum posts (e.g. "cool")
2. Go to the site admin and disable the tags functionality.
3. Go back to the forum
4. 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.