Steps to reproduce:

1. Go to Commons and try to add one category using HotCat. (Note: I shouldn't even see an edit window when only one category is being changed.)
2. NWE opens. The category you tried to add is nowhere to be seen.
3. Type cat by hand, if you can remember the exact name.
4. NWE ate the edit summary, too. Save the page anyway.
5. Disable NWE at Commons until it and HotCat can play nicely together.