The filter used in the plugin is 'the_content,' which is problematic, as it will add the related posts data to custom post types. I added some janky $wp_query->query_vars['post_type'] != 'post' conditional check, but as the_content doesn't receive an argument of the $post, I didn't see any other quick fix.
http://wordpress.org/extend/plugins/possibly-related-recent-posts/
