I know this has been discussed plenty of times, but I got the scare of my life last night.
I was running a sample acrylic item on a scrap sheet of black acrylic that had room on it to fit the piece. I was watching the laser closely since I wanted to check to see if some of the optimizations I made to the vectors was working.
Blower on, air assist on, and the masking on the bottom only. As I was watching it, there were some flare-ups which I usually see on clear acrylic so I wasn't immediately concerned. After a second or two this flare-up didn't go away and got bigger. The masking then caught on fire. I first hit the reset button and got the laser head out of harms way, then turned off the blower and the air assist. I kept the lid shut and after what seemed an eternity the flames went out.
No damage to the laser and I cleaned the lenses and ran the sample (nervously) on another sheet. It worked out fine, no flare-ups, but I called it a night!