On a trip last week, my pack was very full, so I wanted to make as much room as I could. When my first gas canister finally ran out with 3 days still to go, I thought I'd try piercing it and crushing it to make a tiny bit more room available in my very full pack (even after 10 days, I still couldn't fit all my gear back in the pack, and was carrying some stuff on the outside in another tough bag).
I found that my Gerber tool (like a Leatherman) had an old style can piercer on it, so when the gas ran out, I left it on (valve open) for a few more minutes, just to be on the safe side, then pierced the can several times, then left it for a few minutes more to be safe again. Then crushed it between two rocks. Worked very well! I even got all my stuff back in the pack on the very last day, after eating as much food as I could.
Anyhow, I just wanted to say THANKS to the people in this thread who gave me the idea!