After the most recent update I started receiving this error after publishing.
WordPress database error: [Unknown column 'p.user_nicename' in 'field list']
SELECT DISTINCT p.ID, p.user_nicename, MAX(p.post_modified_gmt) AS last_post FROM wp_users u, wp_posts p WHERE p.post_author = u.ID AND p.post_status = 'publish' AND p.post_type = 'post' AND p.post_password = '' GROUP BY u.ID, u.user_nicename
Warning: Cannot modify header information - headers already sent by (output started at wp-includes/wp-db.php:587) in wp-includes/pluggable.php on line 865
Paths modified for security purposes.
It took me a while to track it down and finally with a recursive grep I traced it down to Google Sitemaps...
# grep "p.user_nicename" ./* -R
These were the only mentions for p.user_nicename...