I've been using some of Screwfix's red wall plugs and found they leave a red stain on your fingers. This didn't seem important at the time as I expected it would wash off easily.
Unfortunately I then decided to hang a couple of paintings on a newly papered wall with said red fingers. The red stain transferred to the white, slightly shiny wallpaper. It won't come off.
Management isn't too happy and the paintings are now hanging 100mm higher than intended to hide the stain...