When a user creates a new category, it is not visible in the Control Panel. Despite this, the category still exists and is still able to be used.
Steps to reproduce:
Go to Control Panel
Go to Categories
Add a new vocabulary (Title it "Vocab1")
Add a new category into Vocab1 (Title it "Category1")
The user will not see the newly created category in the category list
Try adding the category again ("Category1")
The system will say that the category already exists.