After upgrading from WordPress 3.4.2 to 3.5, cron has stopped working.
For example, scheduled posts no longer work. (They were fine in 3.4.2).
I looked at the error_log and found this:
WordPress database error MySQL server has gone away for query SELECT option_value FROM wp_options WHERE option_name = '_transient_doing_cron' LIMIT 1 made by _get_cron_lock