I used to run a 7/0 on the back of my K-16 until I noticed the problem and started looking at all my plugs, I will do my best to find one tonight and post a picture, I just have to remember to look. Again, it was just the point, maybe 1/2 mm of the point if that much but you could see the inner non-shiney metal and the outer finished layer.
Just something to look out for.