Spark plug troubles

I have never heard of this before, but it has been a nightmare for me.  An NGK spark plug that was installed about two years ago at the dealership decided to fall apart inside our van last month and ruined the engine.  We had a few unexplained misfires the week before and could not figure out the problem and the mechanic was test driving it when it melted and dropped its core into the cylinder...unfortunately, neither the mechanic nor NGK would accept any responsibility and I  still got stuck with the bill to replace the engine.