If a CO2 cannister failed while packed in someone's luggage, it could have dire consequences for an inflight airplane -- especially if it were near any important instruments. The unplanned explosion of a CO2 cartridge would most likely happen if a drastic change in air pressure were to occur. While the chance of this happening is slight, airlines cannot take the risk, and CO2 cartridges are required to be shipped via ground. Even ground-shipped canisters are regulated as hazardous material and are not permitted in certain tunnels due to the small chance of an unplanned explosion.
CO2 canisters are sometimes confused with nitrous oxide canisters, used in whipped cream dispensers. Inhaling nitrous oxide is illegal, extremely risky and can cause serious health problems, but some people engage in it to get a "high." Inhaling CO2 from a CO2 canister causes death due to suffocation. Any inhaled amount stops oxygen from flowing to the brain. While inhaling either is dangerous, people can mistake CO2 cannisters for nitrous oxide canisters -- ensuring almost certain death or, at the very least, brain damage.
One Shot Only
If you're using a CO2 canister to inflate a flat when road cycling, you have just one chance to get it right. Once the canister is activated, it cannot be reused. If you're out on a long ride or in a race, and are unlucky enough to have multiple flats, you'll need more than one canister -- which means more to carry. Even if you do have more than one but botch reinflating your tire, you'll be left with no fail safe should you get another flat later in your ride. Carrying a mini-pump in addition to or in lieu of CO2, ensures you'll always have the ability to get going again -- provided you have enough tubes, too.
CO2 cartridges are more susceptible to spontaneous explosion when exposed to high temperatures. Keep them in a cool and dry space, away from direct heat and sunlight. Store them upright, and prevent them from rolling around often or bumping into other stored cartridges or objects.