I might have a V3 too then. The bottom is rather stable, although still quite thin, and I bought it a week and a half ago from someone who had just received it from HK.
I used to be a professional software test engineer in another life. So I tried to jog the FSM in the mod's firmware in a systematic way to trigger the problem you're experiencing and determine how it hapens - by screwing / unscrewing the atty with different configurations / presets / buttons pressed. But I honestly could not reproduce the problem, however hard I tried. That's what leads me to believe your own mod may be faulty, but that it isn't necessarily a hardware or firmware design flaw, like the title of your thread suggests.
This said, what happened to you is rather scary, and I'll certainly keep a watchful eye on my own E-Mech. In any case, I always charge my batteries and devices containing batteries in a closed M2A1 ammo box. So if something catches on fire inside it, it won't burn my house down.
