Well, the contacts looked like total shit on these carts. I had to scrape oxidation off some of them (tin colored, not gold, heh). Once a light sanding was done and contacts checked with a multimeter, I was sure the contacts were in good shape. The two or three carts I saw this artifacting on all had EEPROMs, all came from Brazil, all had oxidized contacts. Coincidence? Well, possibly, I guess, but they seemed to have something to do with each other.
I would like to hear someone's opinion on how humidity could affect an EEPROM, because jb143 is right, it is unlikely when thinking intuitively, but as I've stated, I've been witness to such phenomena, um, maybe, heh.
-Rob