I use my iPhone's alarm clock to wake up on weekdays. Twice now, it has failed to go off in the past week or two. When I look at it, it's "Off," but I haven't turned it off. In fact the only time I ever launched the clock app was to set up this alarm, except when I've had to turn it back "On." Of all the bugs, this one really bothers me. I just deleted the alarm and made a new one. Hopefully, this doesn't happen again.