The little grey mark here, near the point of my razor blade, was made by a metal tool – straightedge, probably – brushing against the wallpaper. With some papers, this can happen, and you have to handle the material very delicately. I’m not sure exactly which tool did this, but I’m a little surprised, because I am careful, and because I invested in a pricy magnesium straightedge, because that metal is supposed to not scratch wallpaper.
Luckily, virtually all marks like this will wipe off with light pressure from a damp rag.