The failure mode I've seen 3 times is for the flint to simply disappear. Sounds weird I know, but I've stashed lighters in various places and tested them a few years later -- to find the flint gone. In one case it obviously had turned to powder, the others I don't know. These were kept dry, in a non-climate controlled location like a garage or car glove compartment. So while I carry a lighter and still stash them, I don't trust them, and will rely on waterproof or stormproof matches.

Steve