Spark plug replacement - Fuel consumption?

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I certainly hope your mechanic is not putting 'grease' on the threads of your new spark plugs. I trust you mean an anti-seize product ?

Mechanic is a Mercedes certified tech.
He smeared very little of the correct anti seize paste on spark plug threads.

Today I took a closer look at my old plugs.
They all showed the normal wear and tear signs of use, meaning small amount of black oiliness on threads and some residue on the white ceramic part of the plug.
Almost all plugs were worn as they should except for one of plugs explained below what happened with it.

One of the plugs which was the last one fitted in the back of the engine near the firewall showed some greyish corrosion on the top of the plug.
For you to understand, I have painted grey paint seen in the attached photo.

Mechanic told me he had never seen this corrosion on a spark plug on any car before!?
Mechanic also told me that he replaced the spark plug wire because of the corrosion which was inside the metal hat.
Is it possible it could have been water entering when cleaning engine with a power jet wash causing oxidize/corrosion between plugwire-hat and the top part of the sparkplug?
It was a greyish color of oxidize.

Does anyone know what could have happened with this only one spark plug and the wire?
Please see attached photo.

Mechanic told me my car is solid and drives like a bullet train.
He likes my car very much

Thanks
 

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I went by the shop talking with 3 other senior mechanics, I showed them the sparkplug.
They told me they suspected that the hat of the sparkplug wire wasn't seated properly causing one plug to corrode inside the hat?
I told them we'll keep an eye on this next time I see them, they will remove hat and check the plug if it still occurs?
 

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