I migrated this blog to serendipity almost two years ago, and there have been a few things about it which have always driven me a bit mad. Many of these improved when I upgraded to version 1.4.1 of s9y but a few have persisted, including having two of my sidebar categories not display correctly. Where most of the categories behave as expected, two of of them just didn't apply any filter at all and all my posts were visible on these pages.
I finally sat down today to diagnose the problem and discovered I was missing a record from the serendipity_permalinks table which was stopping s9y from being able to look up the id of the category. It was my craft category which is id 7 - all I had to do was run the following against mysql:
INSERT INTO serendipity_permalinks SET permalink = "category/craft", entry_id=7, TYPE="category";
So now my craft category works fine. If anyone knows how to stop my feeds from showing old articles as new when I edit them but don't update the publication date, I'd be grateful! A bunch of imported posts are still missing their images, would be nice to be able to fix these without polluting my feed.