My site, which has migration as an import component, was starting to throw server errors when trying to view a taxonomy term page. I was able to trace this back to metatag module and the fact the term ID actually no longer existed (the migration creating it had been rolled back and reran, thus causing the tid to no longer be valid.
When metatag module loads the taxonomy term for the current taxonomy overview page it does not check if the lookup actually had a result and passes a potential NULL value into the Drupal API. This will eventually result in an EntityMalformedException: Missing bundle property on entity of type taxonomy_term. I will attach a patch in a minute.