I also think it was an impurity in the steel. It is a good reason to consider Crucibles version of S7 as their powder metallurgy is supposed to stop exactly this type of thing. It makes me wonder how many knives are out there that have the same defect but have not been used heavily enough to find it?

It is another reason to give your knives a serious work out.

I am sure all knife companies have these issues but I don't know many that will replace a second hand modded and polished blade.