I had an ignition coil replaced on cyl. #5 12 months ago.
Several days ago the engine began running rough and the
check engine light began flashing. The diagnostic indicated
a misfire on cyl. #5. I ordered a new coil and when I removed
the old one the spark plug was inside the rubber boot.
The repair shop never screwed the spark plug into the engine block.
I assume the spark was arcing from the plug to the engine block
for the past 12 months!