Tetris (Arcade, Atari)
This game has revisional differences.
Atari's Tetris is the Western counterpart to Sega's arcade game from the same year, and pioneered the concept of two-player simultaneous Tetris.
Besides being widely bootlegged, Atari's version also laid the groundwork for Tengen's Vs. System and NES versions, the latter of which was infamously on the losing side of a licensing dispute against Nintendo.
Two different releases of the program ROMs for both upright and cocktail cabinets are known to exist. The original release is labeled "set 1" and "cocktail set 2" in MAME, whereas the later one is labeled "set 2" and "cocktail set 1". The later release patches the original release's code in to delay an additional 85 clock cycles in between writes to some POKEY sound registers, possibly in order to avoid audio glitches.
These changes were made by directly patching the original ROMs, but the internal checksum used by the game's service mode was not adjusted to account for the changes. Instead, the checksum routine was sloppily patched by inverting a branch condition so that almost all possible checksum values are actually considered "good"; since the game uses an 8-bit checksum here, this means that the later version only has about a 1-in-256 chance of correctly detecting a faulty program ROM.