We just upgraded WP Popular Posts to 2.3 and found that it left the site littered with database errors like this
WordPress database error: [Unknown column 'day_no_time' in 'field list'] INSERT INTO wp_popularpostsdatacache (id, day, day_no_time) VALUES (400, '2012-09-03 14:09:10', '2012-09-03') ON DUPLICATE KEY UPDATE pageviews = pageviews + 1, day = '2012-09-03 14:09:10', day_no_time = '2012-09-03'; Notice: Undefined property: wpdb::$print_error in /wp-content/plugins/wordpress-popular-posts/wordpress-popular-posts.php on line 483
Simple fix to this one – deactivate the plugin and reactivate it.