[zeta0134]

Lots of other games have custom logic in the cartridge. Super Mario 3 and Kirby's Adventure both have a scanline counter in the cartridge that works by snooping the graphics chip while it reads the sprite and background tiles. Because the access patterns were consistent and well understood, it could keep track of where the console was mid-frame and trigger an interrupt, making it much easier to do scroll splits and fancy raster effects. Go check out the final boss from Kirby's Adventure, or the crazy rotating stage in Butter Building, and remember that the NES is doing that with a single background.

Some of this may have been undefined behavior, but the MMC3 chipset involved was produced and manufactured by Nintendo. Whatever the original design was, Nintendo appears to have stuck with that same set of capabilities throughout the console's lifetime, and encouraged their developers to take advantage of all the tricks.