Something to do with the categories database table, but I don’t know what.
I recently installed Rob Sable’s excellent plugin, for Textpattern so that I can add my articles to as many categories as I want.
Just now, I posted an article using Live Writer, and the whole unlimited categories thing stopped working. I’ve checked the database table, and I can’t see anything wrong. My fix was to drop the textpattern_category table, reinstall the plugin and start again.
I liked the idea of Writer, but I like the unlimited categories more. It could be that I need to tweak the RPC script to be more friendly towards the categories plugin, but that’s a job for another time.