I was debugging a buddypress problem and found one in sitewide tags also. Sitewide tags is inserting the initial autosave draft of a post into the tags blog. Create a new post and get off the post title and into the editing are a mu does an autosave of a draft. Also sets the slug for the post.
Sitewide tags is not picking up on this and it's processing this vapor post as a real post. I was looking around in the sitewide_tags_post() function and it would seem to filter out such a thing with
if( $wpdb->blogid == $tags_blog_id || $post->post_status == 'inherit' || $post->post_type != 'post' ) return;
It obviously doesn't. When this autosave draft happens we are not on the tags blog in this fn and I don't know if the post has a 'post_status' of 'inherit' yet. It's definitely *not* catching it here and goes on to happily to save this to the tags blog.