"I ALWAYS turn my phone and any other devices COMPLETELY OFF at important events"
I used to have a Nokia N82 that would wake up from the "completely off" state to sound an alarm. Only removing the battery would do the job.
Fortunately I didn't discover this while attending a concert (or worse: while being one of the performers in a choir).
I'm actually not sure whether Android doesn't do this. Nowadays I always check that there is no alarm set at an inconvenient time before going to silent mode.