A couple of years ago a scented wax burner my girlfriend was using shattered, and the wax poured out of the pot onto the electric burner that heated the wax. The wax quickly caught fire and poured onto the floor. I grabbed the extinguisher, and she called 911. I was able to stop the fire, but I was a little worried about the possibility of the fire re-catching so I didn't call 911 back to stop the fire department from showing up. I needed their expertise on this one.

Anyway, after using the extinguisher it continued to hiss a little. I asked a fireman about this and IIRC he said that quite a number of the cheaper fire extinguishers on the market, especially the "One use Only" disposable extinguishers, are meant to bleed off all of the left over propellant after the safety tab is broken.

I can't remember why he said they did this.