Encountered something new. Had an intermittent miss on my '99 Explorer so I changed the plugs that were in it when I got it. On one of the plugs when I pulled the boot the plug terminal came with it. Just pulled right out of the center of the ceramic insulator. As it happened when I then put a wrenc h on the plug to unscrew it, the insulator snapped off which gave me a good look at the stub of the terminal that was left in the plug. Both the end o f the part of the terminal that pulled out and the remainder in the plug we re well rusted at the junction where they came apart and there's blue/green residue. Looks like the terminal corroded through a while ago.
I'm sure this happens, I've just never encountered it before. What would ca use this? That terminal seems to be pretty well protected from moisture sin ce it's buried in the ceramic and tightly covered by a rubber boot. I would think the high heat would chase away moisture too.