I recall an incident in which the failure of the ELT to go off killed people.
The pilot was killed in the crash, which wasn't severe enough to trigger the ELT (or the trigger didn't work). The passengers survived, but none of them knew to press the red button just to be on the safe side. They had all died of exposure by the time help arrived.
These days when I'm taken on a flightseeing trip over mountains I always check to see where the ELT button is.