Clearing the cache didn't work because apparently I forgot to remove the calendar event when deleting a topic.

The fix:

scripts/doexpirethreads.inc.php

About line 238 you'll see:

$html->send_redirect(

Right before that, add this:

Code
        $query = "
                delete from {$config['TABLE_PREFIX']}CALENDAR_EVENTS
                where TOPIC_ID = ?
        ";
        $dbh->do_placeholder_query($query,array($Number),__LINE__,__FILE__);