Please note, the following is a quoted error message:
Database error: [You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near ‘,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,]
SELECT DISTINCT ID, category_id, cat_name, category_nicename, category_description, category_parent FROM blog_categories, blog_post2cat, blog_posts WHERE category_id = cat_ID AND post_id = ID AND post_id IN (,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,)
Warning: Invalid argument supplied for foreach() in /path/to/wordpress/wp-blog-header.php on line 478
If your blog has suddenly been blessed by the above error message, then your host has just updated their copy of PHP to v4.3.10, but forgot to update Zend Optimizer. More information on this issue is available here.