Last modified: 2009-09-10 03:37:46 UTC
Having the 'Has default form' property set in a Category:... page not only causes the pages in that category to have an 'Edit with form' tab, but also the Category:... page itself. It doesn't make sense to edit a Category:... page with a form that is intended for its member pages though.
I don't know if this has been introduced with version 1.8.2 ('You can now set a "Has default form" value directly for pages, instead of going through a category or namespace'), or if it has been like that before - in any case I consider it a bug.
This problem was indeed introduced in version 1.8.2, and it's now been fixed in 1.8.3.