The title sais it, if you accidentally set the "Parent Forum" to the current forum being edited, it disappears from the list of forums effectively becoming a dangling forum.

The database still has it, with parentid being equal to forumid. Here's what it looks like:

+---------+---------------------------------+--------+----------+
| forumid | title                           | is_cat | parentid |
+---------+---------------------------------+--------+----------+
|       1 | Information                     |      1 |        0 |
|       5 | Help & Support, Troubleshooting |      0 |        4 |
|       3 | News and Announcements          |      0 |        1 |
|       4 | Questions & Answers             |      1 |        0 |
|       6 | Feedback                        |      0 |        6 |
|      14 | Feedback                        |      0 |       14 |
|      13 | Support                         |      1 |        0 |
+---------+---------------------------------+--------+----------+

And what is shown in the admin is attached. Both "Feedback" forums are invisible/dangling.

P.S.: Offtopic/Security: wpforo.com doesn't have a valid cert!