'death' of the rubber on an old pencil. usually discovery of this is by making nasty, hard to remove streaks all over your "masterpiece" when trying to erase.......

i nowadays get suspicious when the eraser looks dark...i start fingernail poking, checking the 'elasticity'; saving me the effort of industrial cursing!!

(maybe its' just an 'old peoples' thing!!?? i forgot about "paperless environment"!!)