Hello,
For a code:

$query = new EntityFieldQuery();
$resFirstName = $query->entityCondition('entity_type', 'node')
            ->entityCondition('bundle', 'user')
            ->fieldCondition('field_user_first_name', 'value', $str, 'RLIKE')
            ->range(0, 15)
            ->addTag('node_access')
            ->execute();

when site's user isn't Admin I get this error:
"PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'field_data_field_user_first_name0.nid' in 'where clause': SELECT field_data_field_user_first_name0.entity_type AS entity_type, ..."

If I remove ->fieldCondition('field_user_first_name', 'value', $str, 'RLIKE') or ->addTag('node_access') - no error.

Thanks