I've got a Surefire 9P light I've been using for... I think about fifteen years. Couple of days ago, in the middle of lighting something, it went off. "Crap, guess the lamp finally went."
No, tried a new lamp and no good. Checked the batteries, and they showed, if anything, a bit above full charge. Which is odd, as they've been in there for months, and it's been used. So put in new batteries, and there is light!
At this point I don't know if one of the batteries went weird and cut flow, or if they all did something, and I really don't know, other than switching one at a time, a way to find out. I may mess with it later.