Oh wow, I have surely learned the hard way to function test everything that I pack for a camping trip. I think that was my second packing lesson after learning the value of a packing check list.
None the less, looking my back some of early packing failures are more memorable than any of my successes. Plus my packing failures did teach me to be more self reliant and helped develop new skills.
Today, my planning checklist starts with the assumption that I will arrive at the trail head or campsite after dark, in rain and heavy fog. Therefore, the lights on my person (at least three or four) need to have fresh batteries and spare batteries readily at hand. The lights are usually a mix of incans for throw and fog piercing ability and LEDs for personal and immediate area use. If I arrive while it is bright and sunny, at the worst I have a couple of extra pouches on my belt and a gore-tex rain jacket with bulging pockets.