This has been happening since I upgraded to WordPress 2.9.1. When drafting a post, sometimes upon pressing "Save Draft" I notice that additional categories have been selected, even when I did not select those categories in the first place.
i.e. I write a post, select category "Rants" and after pressing "Save Draft" or Publish, suddenly I see that the "uncategorised" and "ramblings" categories have also been selected. Is this a feature or has my blog been compromised?
PS: Edit revision says I'm the only editor of that post.