It looks like that plug was loose allowing combustion gases to leak. That can lean out the cylinder and cause the plug to burn up like that. I would check all the plugs and replace any loose ones along with the coil boots. Put dielectric grease in the boots and you should be good to go.

 

I'd bet money the wiring to the connector has a short. Maybe even inside connector . From what I've seen and read , I would replace all the connectors, since your already goin back in anyway. You can find good ones cheap at scrapyard, $2.50 ea. If nothing else, you can rule that out. Just got done replacing all 8 plugs, boots 2 coils and 2 connectors, all I had, but would have replaced all 8 if I could. At least the back 4 just to be practical.

 

Almost forgot. While your head and shoulders in it, take the opportunity to THOROUGHLY clean plug ports, including threads. I used Berryman B-12 chemtool, (works faster) some clean rags and screwdriver. I recommend bottle brushes. If you had an exhaust leak there, be damn sure the bottom of the port where the plug seats is SPOTLESS! Get out your mirrors and pocket lights. You'll be glad you did when you're done.
Going back together, as cleanly as possible, use new boots with dielectric gel at both ends of coil/ boot assy. a dab at the bottom and a thin film at the top.
Contanimants can and will wreak havoc in your entire ignition system. Next comes O2 sensors and cats if problems prosist.