It appears the "Categories" filtering is a little too inclusive.  It includes documents that are outside of the defined root for the module.

Steps to reproduce:

1) Within the true root of DMX, create two subdirectories (collections) under the true root of DMX ("Collection1" & "Collection2").  Make sure that your test user has at least view access to both collections.

2) Create a new instance of the DMX module on a DNN page with "Collection1" set as the pseudo root.

3) Add one document called "Doc1" to the "Collection1" collection and assign those documents to a category (Category1).

4) Add one document called "Doc2" to the "Collection2" collection and assign it to the same cateogory (Category1).

5) Go to the module instance that has "Collection1" set as the root (the pseudo root).  Click on the Categories link in the DMX menu.

6) Click "Category1".  You will see both "Doc1" and "Doc2".  "Doc2" should not show up because it's outside of the pseudo root.

7) While you are still in Category view, if you click the collection link "Collection2" for document "Doc2", you will get this error, "A critical error has occurred. Conversion from type 'DBNull' to type 'String' is not valid."

In my opinion, the categories filter should only show documents that are within the defined root of the module instance.