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As soon as the metatags module is activated, the 'metatags' property is added to every entity in hook metatag_entity_load().
In Drupal 7 entities are also used for configurations, e.g. taxonomy vocabularies or Entity API configs like Profile2. On such entities the metatags property does not make any sense. Furthermore it makes many features overridden, if you activate the metatags module once the features have already been exported.
Attached patch adds a check in metatag_entity_load() on whether the current entity type supports metatags. Additionally this patch avoids unnecessary metatag_metatags_load_multiple() calls when the entity type can not have metatags anyway.