To move a course or category to a different category, you would go to the category management screen, select the checkbox next to the course/category you want to move, then select the destination from the "Choose..." dropdown, and finally click the Move button.

If you attempt to select the destination from the "Choose..." dropdown before you have selected a course/category to move, you can't. The dropdown is disabled and the cursor changes to the "no" symbol (circle with a line through it). This is good.

However, the Move button is not disabled. It is possible to not select anything and simply click the Move button. Doing so results in an unfriendly error:

A required parameter (movecategoriesto) was missing

More information about this error

Debug info:
Error code: missingparam
Stack trace:
line 463 of /lib/setuplib.php: moodle_exception thrown
line 545 of /lib/moodlelib.php: call to print_error()
line 310 of /course/management.php: call to required_param()

Expected result: The Move button should be disabled the same way the "Choose..." dropdown is disabled.

Actual result: The Move button is not disabled and clicking it results in an unfriendly error.