Drupal 8 "Mismatched entity and/or field definitions"
Updating modules in Drupal 8 is similar to updating modules in Drupal 7 - the drush pm-update (drush up) command still works, but there's a new "gotcha" that you need to look out for when updating modules that work with entities.
Sometimes, after updating modules that interact with entities, you'll see a "Entity/field definitions: Mismatched entity and/or field definitions" error on your site's status report (admin/reports/status).