When I create course categories, the second menu does not expand and show the sub-categories. This happens only when number of sub-categories reach 3 sub-categories under a top category.
To replicate the problem please create 2 top categories and create 3 sub-categories for each top category.

Screenshot:
https://moodle.org/pluginfile.php/143/mod_forum/post/997677/subcategory.jpg

Moodle 2.5+ (Build: 20130530) on Ubuntu 12.04.2 LTS