The skin (normally very thin aluminum) of the containers are quite thin, I have seen two discharge due to puncture. Fortunately I owned neither of them and they were not in my car or within 20m of me at the time.

I now always check to see that my Bear Spray is securely inside a pocket of my pack before stowing it in a car, not dangling on a belt to be; dropped, sat on or have something dropped on.