I get this when trying to edit an existing node with a populated entity reference w/ an autocomplete widget:
Error message InvalidArgumentException: Property revision_id is unknown. in Drupal\Core\Entity\Field\Type\EntityReferenceItem->setValue() (line 84 of /Users/webchick/Sites/8.x/core/lib/Drupal/Core/Entity/Field/Type/EntityReferenceItem.php). The website has encountered an error. Please try again later.
This is the very definition of "Needs tests" :P