All ROMs can suffer from the effects of EM (
electro-migration ).
They can suffer from impurity ions as well. Copper and
sodium are real bad.
If you wait long enough, sold-state diffusion will get you
anyway. Not a real issue with many old things (say, Trajan's
Column or Cleopatra's needle) but when you are dealing with
fine structures (on the order of micrometres even in the 1970s
I would guess) I would guess a few hundred years is enough
to introduce the occasional error.
Best to back everything up right now (and refresh onto
new media as required) :-)
Antonio
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Antonio Carlini arcarlini(a)iee.org