> Pen tapping or scribbling on disc
For pens with metal tip (Ballpoint / Rollerball), all you have to do is exert light pressure on the reading side of the disc surface, which can easily create an indentation right down to the data layer causing deformation to the data layer thus creating irreparable damage.
> Pencil tapping or scribbling on disc
Pencil core consist of a blend of graphite and clay compound. Most common use of the pencil grade is HB, which is a harder grade and when exerts pressure on the reading side of the disc surface, there is high risk in which the created indentation is deep enough to cause irreparable damage. Alternatively, softer tip pencil (i.e. 2B and above) the risk of indentation damage beyond repair reduces considerably.
> Placing disc onto unprotected surfaces
Not putting the disc into its original case or sleeve is a common habit and the most critically overlooked element as result of this action is the size of dust granules on the placement surface. If the granules are large enough and sufficient forces are pressed downward on top of disc plus horizontal movements, indentation damage can easily occur; therefore risk of damage beyond repair can be quite significant on this particular scenario.
Friendly note: Disc damage from chewing, biting or used as Frisbee, ones hope of recovery is pretty dim.