I have seen this on Focus ( zetec) and Mondeo (duratec) that have Motorcraft spark plugs before. The Motorcraft plugs seem to rust in exactly the same place every time - just were the metal is the thinnest, resulting in a sheared plug. It is often on the first plug change the car has had. My solution has always been to fit NGK spark pugs, they seem to have a thicker jacket and resist corrosion better. I always put the water ingress down to valeters being over enthusiastic- in conventional cars that is