Haio, looks like we're getting a few errors in the Moodle cron.

<snip>
... started 06:30:06. Current memory use 16.4MB.
PHP Notice: Undefined variable: log in /var/www/vhosts/moodle.org/html/calendar/lib.php on line 3207
</snip>

and

<snip>
Execute scheduled task: Sync external blogs
... started 06:28:21. Current memory use 22.7MB.
... used 0 dbqueries
... used 2.1934509277344E-5 seconds
Scheduled task complete: Sync external blogs
Default exception handler: The operation timed out while waiting for a lock. Debug:
Error code: locktimeout

• line 450 of /lib/classes/task/manager.php: moodle_exception thrown
• line 66 of /lib/cronlib.php: call to core\task\manager::get_next_scheduled_task()
• line 61 of /admin/cli/cron.php: call to cron_run()

!!! The operation timed out while waiting for a lock. !!!
</snip>

These were shown on the moodle.org server.

It looks like the waiting for lock error is occuring randomly on different jobs in the cron, I also see it mentioned for a clean up old sessions job and a couple of others in the logs as well.