Probably not an issue for most, but the other day I pulled a Bic lighter out of a small pullout bag in a pack and the wheel was loose, no flint.  Weird because I never use it and it was still full of fuel.  Turns out the wheel had been slightly turning over time, grinding away at the flint, and eventually ground it all away.  No sign of spark.  Granted, I did pull off the kid safety spring because it is annoying, but in the future I'll leave the safety on.  Also a lighter case would prevent this, but at added cost and weight.  Ultimately Bics are so cheap light and small it's one is none, 500 is one rule for me here, but just thought I'd share in case you have a Bic in a ruck somewhere with no flint.

I just cleaned out and redid an old camping backpack and found the 10 year old Bic lighter was bad.  Tried to light it and the flint just puffed into a small cloud of dust with no spark.   Still had gas though, didn't leak.  Was stored in a ziplock bag, in a backpack, in a climate controlled house.

I figured a Bic would last forever but apparently I was incorrect.  Also had an Energizer Lithium AA leak and ruin a flashlight in the same bag.

This is one of the reasons I don't like "back up" kits. They tend to be neglected and when you finally use them, shit is missing or broken. Also, you never use these "backups" so you only use them when you absolutely have to in an actual emergency, which is a hell of a time to meet your new kit.

That said, I do still retain a small survival kit based off of the round 8floz spice containers. It rides in a knife sheath and gets used as a primary kit for bushcraft and what not.

The flints can oxidize over time

The catch 22 is that they are coated at the factory against the corrosion which is rubbed off with the striker when first used. So when you test it to make sure it works it opens it up to corrosion in the future

Living in the desert it seems like everything goes bad faster with the heat.
I do regularly buy a 4 pack of lighters at the check out stand and just keep piling them up with the batteries.

Even my zippo flints have crumbled even though I had them in the little carrier and vacuum packed. No rough treatment.

I ran into issues with Bics too and prefer fire steel in my packs.  They last thousands of strikes and are more durable.  They take about the same amount of time to get a fire lit with my tinder kits.

I've mostly switched over to Scripto Electronic lighters due to the flints in Bics going bad. I have a couple that are ~10 years old that still light every time. Also, the child lock in them is on the gas button, not the striker wheel, so they don't tend to get accidentally depressed and let all the butane out like Bics can.