看来term_exists() 正在为已存在的术语返回NULL：

$name = "כל האזורים"
$taxonomy_name = "geographictermsisrael"
$args = array("slug" => "everything", "parent" => 0)

var_dump(term_exists($name, $taxonomy_name, $args)); // Returns NULL (not 0)

if ( !term_exists($name, $taxonomy_name, $args) ) {
    $foo = wp_insert_term($name, $taxonomy_name, $args);
    var_dump($foo); // Returns the array below
}

object(WP_Error)#4193 (2) {
  ["errors"]=>
  array(1) {
    ["term_exists"]=>
    array(1) {
      [0]=>
      string(77) "השם או מזהה הכתובת כבר קיימים עם ההורה הזה."
    }
  }
  ["error_data"]=>
  array(1) {
    ["term_exists"]=>
    string(3) "699"
  }
}

mysql> select * from wp_term_taxonomy where taxonomy=\'geographictermsisrael\';
+------------------+---------+-----------------------+-------------+--------+-------+
| term_taxonomy_id | term_id | taxonomy              | description | parent | count |
+------------------+---------+-----------------------+-------------+--------+-------+
|              738 |     699 | geographictermsisrael |             |      0 |     0 |
+------------------+---------+-----------------------+-------------+--------+-------+
1 row in set (0.00 sec)

mysql> select * from wp_terms where term_id=699;
+---------+---------------------+------------+------------+
| term_id | name                | slug       | term_group |
+---------+---------------------+------------+------------+
|     699 | כל האזורים          | everything |          0 |
+---------+---------------------+------------+------------+
1 row in set (0.00 sec)

mysql> select * from wp_term_relationships where term_taxonomy_id=738;
Empty set (0.00 sec)