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Oil or coolant collecting in the spark plug well, where the coil and plug sit. The well is a deep tube in the cylinder head, sealed at the top by the cam cover gasket and at the bottom by the plug itself, and anything that leaks into it has nowhere to drain.

Oil is the common case and it destroys coils. A failed cam cover gasket or plug tube seal lets oil fill the well, where it attacks the coil's rubber boot until the spark tracks to earth through the softened insulation instead of jumping the plug gap. The result is a misfire on one cylinder that returns weeks after a new coil is fitted, because the oil is still there.

Coolant is rarer and more serious, pointing at a head gasket or a cracked head, and it will corrode the plug and the coil terminal. Look into the well before replacing a coil — a torch shows it immediately. Clean and dry the well and replace the seal, because a new coil into an oily well fails exactly as the last one did, which is the single most repeated mistake in misfire diagnosis.