The debug room from Mega Man ZX is also present in this game. The room can be accessed with the Action Replay code, 22178FF8 00000000 (USA) or 02165044 00000000 (EUR).
If you press the Transform button while you're currently in Siarnaq form, his animation used when the transformation icons appears briefly includes a frame of Model PX from Mega Man ZX.
The game has unused sprites and mugshots of 10 unnamed characters: 4 women (2 being nurses), 3 men and 3 kids. The only instance one of them (the sprite from a nurse) appear is in the Secret Disk O06 from the game's database.
The game has several unused life bar icons: a Model HX and a Model O for the player's life bar, and a Serpent logo and an O for boss life bars.
The mugshots of the Biometals Model H, F, L and P from Mega Man ZX are present in the game, but as they have no lines in Mega Man ZX Advent, those are unused.
Despite Model X and OX not being playable and their graphics being (mostly) removed, their palettes still remain. The former is not the same palette that is used by Model X in one of the late game cutscenes.
While this graphic technically gets used during the Japanese titlescreen animation, the colors are not visible. These colors were used to visualize the "flow" of the animation as the palette is updated.
The US version uses English voice acting in cutscenes, and for battle cries. The European version removed the voice acting for cutscenes (except in FMVs), but left the battle cries.
The Model A cutscene and Credits music tracks were changed in the international versions, and lack vocals.
