I have the "Catagory Visibility" 0.31 plugin.
When I hide a catagory then all the posts in that catagory are no longer to be found via clicking the Catagory link itself in the catagory list.
When I UNhide it from the Frontpage (or other) then it returnes to normal.
When the Error occurs I get
Sorry, but you are looking for something that isn't here."
But if I do a search in the search bar I can easaly find a post in that Catagory.
This wasn´t a problem yesterday (only had this site up for 3-4 days, first time WP blogger).
Only thing I´ve changed since yesterday is that I added users and added one link in the sidebar!