The Protogunner has an unused animation for firing its cannon while jumping. It doesn't even animate in that state in-game! Ta Division 六 for pointing it out, and to Random Talking Bush for ripping the sprites.
Unused and partially-unfinished conversions of the MV-280B jeep into the MV-280C introduced in this game. Ta Division 六!
A find from Division 六: when the Mars People hold Morden in Mission 2's cutscene, their tentacles only use two frames of animation, but an extra four frames go unused.
Mars People beckoning the player onwards like the ally Morden soldiers. These graphics are loaded during the first screen of Mission 5, and are even visible off the side of the screen if the Shrunk Screen Mode dipswitch is on - cheers to 이지우 (Lee Jiwoo) for this info! Only the second animation is visible, so the first one probably isn't animated correctly.
These frames are stored at the end of the infected soldiers' landing animation and are minor edits of their idle animation. Likely a transitionary animation they forgot to implement. Cheers to Division 六 for this.
The sewer areas in Mission 3 have static water baked into the background, but are overlapped with animated water instead. Ta to The Huntress for this find.
This animation is used, but triggers very rarely: a Hunter Walker inverting itself from walking on the ground to the ceiling. The animation was later used in Metal Slug Defense and Attack. Cheers to Division 六 for pointing this out to me, and RandomRebelSoldier for ripping it:
The Invader UFOs have an unused animation for phasing in, but they just enter from off-screen in-game. Ta Division 六 for pointing it out, and to RandomRebelSoldier for ripping the sprites.
This wrecked missile is supposed to appear at the end of Mission 2 when the train boss crashes against the wall, but the missile always explode before then, leaving it unused. Ta Division 六 for this.
The Flying Core in Mission 3 will fire junk as projectiles, resorting to shooting fish if you dodge it three times; these crabs were likely meant to be among them, but go unused. Cheers Division 六 for ripping them and that factoid!
Big ups to Division 六 yet again! Mission 4's boss has incomplete sets of shrunken body parts, which are never seen in-game.
This explosion effect was intended for its mucus projectile, but reuses the sprite from its other projectile attack.
When the Sea Worm is destroyed, only three versions of its destroyed body parts are used, leaving all these alternate angles totally unused.
This sprite of the Invader Queen is used after its first form is defeated, but is mostly obscured by its cocoon. Ta Division 六!