'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"!!)
my mind went blank, i made an error, tried to erase; but was tortured by eraser rot!!
eraser rot made my notation look like hell!! i couldn't remove the fuck damn marks!!
all the 'kool kats' @ u.d. HATED my concept of eraser rot!!