I get the same feeling, now, with my wakeup alarms. Long ago, I gave up on my weak bedside clock radio. But twice this past month, I slept through the moderately-loud cell phone alarm, and even the powerful digital egg-timer. Double coverage, failed!
The egg-timer's use was once restricted to critical situations -- like interviews and presentations -- but gradually became overused during intern year. Now, resistance has developed.
Fortunately, in both cases of alarm resistance, lateness was avoided -- through frantic, accelerated morning routines and a bit of freak subway luck. Next time I may not fare so well. New, out-of-the-box approaches may be required to confront this problem.
I could also try sleeping regular hours... but that's too unsettling to contemplate, for now.