Pulled a switch to replace it and noticed that

(a) the switch brought one black wire with it but left two other wires
(one black solid, the other white stranded -- the bare ends were clearly
visible) in the box;

(b) the temperature in the box seemed high -- decidedly warm;

(c) there was a charred black mess that seemed to be "cooked" insulation
tape with a small area of something red and hard at the end where the
wires went in and the end of a screw sticking out of the other end of
this mess.

As I cut away the insulation tape, it became clear that this was a
"fried" wirenut, and that what had looked like screw threads was the
wire spiral that was gripping (or was supposed the gripping) the conductors.

The first time I ever saw a wirenut, I thought "What kind of a Rube
Goldberg kludge is this?" but I now understand that they are NEC
approved. So what would cause a wirenut to "fry" like this? Note: I had
never noticed any problems with the lights that the switch was
controlling, or with any other circuits fed through the wires being held
by that wirenut.

BTW, I think the white stranded wire must have been the ground wire to
the metal "frame" of the switch. The wirenut fell off the ground wires
before I had a chance to take careful note of where the other end of the
white pigtail went.

Perce