In production, some tags, such as Guardians of the Galaxy (Movies) are missing their inherited meta tags. Until these meta tags are fixed, tags like Guardians of the Galaxy will be treated as having 0 first-class meta tags, resulting in a lot of false negatives for the crossover check. (Because currently, we check whether one fandom's first-class meta tags are a subset of the other's.)
The crossover check should iteratively and/or recursively compute the missing inherited meta-tags to fix the check (at least until the meta-taggings table is fixed).
1. To create a fandom with a missing inherited meta tag, create four canonical fandom tags. I'll call them Fandom A, Fandom B, Fandom C, and Fandom D.
2. Edit Fandom A and add sub tags Fandom B and Fandom C.
3. Edit Fandom D and add meta tags Fandom B and Fandom C.
4. Edit Fandom C and remove Fandom A from its meta tags.
5. Keep refreshing the edit page until Fandom A disappears, then remove Fandom D from its sub-tags.
5. At this point, Fandom D should have Fandom B as a direct meta-tag. Because of the meta-tag issue, however, it should be missing the inherited meta tag Fandom A. You can check this by creating a work with Fandom D, and checking whether it shows up when you add Fandom A to "Other tags to include."
6. Try tagging a work with Fandom D and another unrelated fandom. The work should be marked as a crossover, but as a result of this bug it won't be.