ive had the same thing. I had a nice response typed up, then my laptop decided it was time for a restart for some updates.
mine wasnt due to being under purged, or the nail too hot. I somehow filled my dome with propane (yes I use a propane torch) and when I touched the dab on the nail, it made a little fire, turning my dome, and bong, and all my reclaim black with carbon from the flame
ive smoked severely under purged butane on purpose before and it didnt even flame up, despite cracking and sizzling like no tomorrow.
now that I think about it... I think its only happened with this side dome thing I had.. because I had to heat the nail WHILE the dome was right there, so yeah, it filled the dome with gas. I think this is completely different from what you had.