Running MySQL 5.1, Apache 2.2, PHP 5.2, shared hosting.
Question2Answer query failed:
INSERT INTO qa_userbadges (awarded_at, notify, object_id, user_id, badge_slug, id) VALUES (NOW(), 1, 75, NULL, _utf8 'nice_answer', 0)
Error 1048: Column 'user_id' cannot be null
I tried "Recreate badges" and "Recheck badge awarding" with box checked ("Remove all badges from database first"), but this didn't fix it.