Sometimes you'll get a cart that plays fine, but the graphics are screwed up. The Nintendo uses two serperate ROM chips in the cartridge - one for the game program (PRG ROM), and one for the graphics (CHR ROM). They have their own sets of pins. (Which is also why the carts have so many pins...). If the game's program ROM is making good contact, while the graphic ROM isn't, then the game will still play, but the graphics will have odd lines through them (data bus problem), or be composed of the wrong tiles (address bus). In many cases, a good cleaning helps, but occasionally I run across a cartridge that is too badly damaged to save.
-Ian
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