PL/SQL :: Difference Between CROSS JOIN And Comma-notated Cartesian Product?

Oracle version: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit
OS: Linux Fedora Core 17 (x86_64)

I was practicing on Recursive Subquery Factoring based on oracle examples available in the documentation URL....I was working on an example which prints the hierarchy of each manager with his/her related employees. Here is how I proceed.

WITH tmptab(empId, mgrId, lvl) AS
(
    SELECT  employee_id, manager_id, 0 lvl
    FROM employees
    WHERE manager_id IS NULL
    UNION ALL
    SELECT  employee_id, manager_id, lvl+1
    FROM employees, tmptab
    WHERE (manager_id = empId)
[code]....

107 rows selected.

SQL> However, by accident, I noticed that if instead of putting a comma between the table names I put CROSS JOIN, the very same query behaves differently.That is, if instead of writing

UNION ALL
    SELECT  employee_id, manager_id, lvl+1
    FROM employees, tmptab
    WHERE (manager_id = empId)I write
. . .
UNION ALL
    SELECT  employee_id, manager_id, lvl+1
    FROM employees CROSS JOIN tmptab
    WHERE (manager_id = empId)I get the following error message
ERROR at line 4: ORA-32044: cycle detected while executing recursive WITH query

I remember, oracle supports both comme notation and CROSS JOIN for Cartesian product (= cross product). For example

SQL> WITH tmptab1 AS
  2  (
  3      SELECT 'a1' AS colval FROM DUAL UNION ALL
  4      SELECT 'a2' AS colval FROM DUAL UNION ALL
  5      SELECT 'a3' AS colval FROM DUAL
  6  ),
 [code]....

SQL> So if both comma notated and CROSS JOIN have the same semantic, why I get a cycle for the above mentioned recursive subquery factoring whereas the very same query works pretty well with comma between the table names instead of CROSS JOIN? Because if a cycle is detected (ancestor = current element) this means that the product with CROSS JOIN notation is generating some duplicates which are absent in the result of the comma notated Cartesian product.