When trying to run update.php for simultaneous updates of both the entity and og modules (entity from 7.x-1.0-beta7 to 7.x-1.0-rc3; og from a very old 7.x-1.x-dev to 7.x-1.4), update.php hit a fatal error -- and then trying to access any page on the site triggered the same fatal error.

The fatal error itself is coming from the og module (see #1543696: PHP Fatal error: Unsupported operand types in modules/all/og/includes/og.info.inc on line 29). However, the root of the problem appears to be the entity_metadata_convert_schema() function in entity.info.inc. That function is calling drupal_get_schema($table), which is incorrectly returning $schema=false -- the new table does exist, but drupal has not yet had a chance to update its schema cache. So og is being asked to provide entity metadata for tables that it knows exist, even though the entity base class is saying that the tables don't exist.

I believe that the function call needs to be changed to drupal_get_schema($table, TRUE) to ensure that the function is using valid information, even when being called in the middle of an update. Otherwise future entity updates also risk getting getting stuck in a chicken-and-the-egg loop whenever two modules need to be updated simultaneously.