One example of this is bug 109718.

Here's another example:

Say, I navigate to the "Categories" tab in the "content" content
section in order to delete the "Education" category:

    |select category_id, name, default_ancestors
    |from cat_categories
    |where name = 'Education';
    |
    | category_id |   name    | default_ancestors 
    |-------------+-----------+-------------------
    |         100 | Education | 61/100/
    |(1 row)


The "Education" category has the following three subcategories:

    |select
    |  cccm.category_id, related_category_id, cc.name as related_name
    |from
    |  cat_category_category_map cccm
    |inner join
    |  cat_categories cc on (cccm.related_category_id = cc.category_id)
    |where
    |  cccm.category_id = 100;
    |
    | category_id | related_category_id | related_name 
    |-------------+---------------------+--------------
    |         100 |                 101 | Arts
    |         100 |                 102 | School Time
    |         100 |                 103 | Literacy
    |(3 rows)


Before the deletion, I have 43 categories in cat_categories:

    |select count(1) from cat_categories;
    | count 
    |-------
    |    43
    |(1 row)

After the deletion, I have 42 categories.

    |select category_id, name, default_ancestors
    |from cat_categories
    |where name = 'Education';
    |
    | category_id | name | default_ancestors 
    |-------------+------+-------------------
    |(0 rows)

The children of "Education" have been correctly unmapped:

    |select
    |  cccm.category_id, related_category_id, cc.name as related_name
    |from
    |  cat_category_category_map cccm
    |inner join
    |  cat_categories cc on (cccm.related_category_id = cc.category_id)
    |where
    |  cccm.category_id = 100;
    |
    | category_id | related_category_id | related_name 
    |-------------+---------------------+--------------
    |(0 rows)


However, the "default_ancestors" column of the three ex-children of
"Education" has _not_ been updated:

    |select category_id, name, default_ancestors
    |from cat_categories
    |where default_ancestors like '61/100/%';
    |
    | category_id |    name     | default_ancestors 
    |-------------+-------------+-------------------
    |         101 | Arts        | 61/100/101/
    |         102 | School Time | 61/100/102/
    |         103 | Literacy    | 61/100/103/
    |(3 rows)


The correct value of "default_ancestors" for all of the three
categories should be '101/', since none of them has ancestors any
more.