That exactly happened to me, #2 center rear cylinder, twice. It's good to use silicone dielectric grease in the boot, make sure the sparkplug socket wrench is clean and your hands too. I cleaned out the boot inside, greased, new plug, good so far, we will see after winter. I think dirt on the plug insulator, dampness cause the arcing, it's an easier path than sparking inside the engine!
The grease should seal it out. I actually gapped the plugs a little tighter than specified .050 instead of .052. They will spark easier, I think, and the voltage will stay inside the plug! I get the same exact tan rust/heat stain.
