After installing Quick Cache, I discovered my daily database backup (by the DBC Backup plugin) was not running.
In quick-cache/includes/functions/extend-cron.inc.php, the array being passed to function ws_plugin__qcache_extend_cron_schedules() is being discarded, and only the "Every 5 Minutes" schedule is being returned. In my case, the passed array included a schedule for DBC Backup - and it was being discarded.
This Codex shows how to add a new schedule without discarding any schedules which might have been created by other plugins.