I've encountered similar variations to this problem.  The problem is that
multiple browsers get confused about what's in their lists vs. what's
defined in the system as a whole.  Adding something in one browser can
confuse another browser which THINKS it sees the whole list, but something
was just added to the list.

I posted a fix for a couple of these problems ages ago -- I specifically
dealt with classes and message categories.  But it really needs a Browser
overhaul.

The short term fix is to use update (in the Class Categories pane) when
you've changed something in another browser.

Mark

--------------------------
Mark Guzdial : Georgia Tech : College of Computing : Atlanta, GA 30332-0280
(404) 894-5618 : Fax (404) 894-0673 : guzdial at cc.gatech.edu
http://www.cc.gatech.edu/gvu/people/Faculty/Mark.Guzdial.html