I was cutting a job yesterday from 1/16" thick Delrin and it caught fire. First it flared up and dripped beneath the grate. From there, it caught fire beneath the grate and fed itself from the dripping sheet above. This all took maybe 10 seconds while I was standing next to the laser looking a different direction. The first thing I did was open the lid and attempt to blow it out with my mouth but all that did was feed the flame with more drips. 2 seconds later, I reached down and grabbed the CO2 fire extinguisher that's been sitting there for about 10 years and gave it a tiny blast. After about 15 minutes of cleaning the melted Delrin from the grate and changing out the aluminum foil I keep beneath it, I was cutting the job again. Luckily, there was no damage to the laser except some slag stuck on the grate. I'm surprised because the flame was well above the grate and about 6" in diameter. Having a powerful fume extraction system probably helped here beause the flame was going sideways towards the exaust ports.
So... had I not spent the extra hundred bucks or so way back when, I'm sure the damage would have been pretty ugly. Being that it was a CO2 fire extinguisher made for very little cleanup and allowed me to immediately move forward with the job.
If you don't have a fire extinguisher next to your laser, I suggest you do it now.
I also keep another about 20 feet away by the door in case things get out of hand.