Has anyone heard of CO2 cartridges exploding?

12grams are pressure vessels holding liquid co2. Above 87F, no amount of pressure will keep co2 in a liquid state though, and the stuff will just start boiling into a gas and filling the container its in. Larger tanks have what are called burst discs, which are supposed to fail at about 1500psi to vent the co2 out, before the tank wall ruptures itself (which usually happens around 2500psi). 12 grams however have no such feature, and they can explode like a grenade, or more likely, fail at the end where the metal is thinnest for the piercing by an inflator or airgun valve. This however does require leaving them someplace they can overheat. Sitting in a car with the windows closed, or on a picnic table under the sun, or more foolishly, toss some into a bonfire from a safe vantage point (like behind some armor plating and watching it on a CCTV).

I myself paintball too and have had burst discs fail from forgetting a tank in a sunny place, so I'm sure a 12 gram could fail from sitting in the sun also. I've fortunetly never left them in the sun long enough to find out though.