Well, I've never cleaned my button, but it is indeed very dirty around the points of contact between it and the button ring. IT"s sort of tough for me to tell whether it's carbon buildup, or just dirt, but I'll assume it's carbon. I'll clean it and report if it happens again.
I don't think it's the Spheroid either. The amount of heat I got was insane, and it was on the mod, not the atty. Both times it happened, the point of heat was right at the top of the ELA, slightly below the locking ring. It was almost like the heat 'seeped' directly out of the crack into my finger. Longer than just a "spark"... there was a distinct "heating-up" period, that lasted about half a second, and was gone as soon as I let go of the button. Followed by a string of curse words.