There's a secret ending in the game. If you talk to a man atop a climbable wall in stage 3, he will offer you a chance to partake in the Battle Arena. Doing so will warp you to an arena playing a remix of Vampire Killer from Castlevania, where you fight a three bosses:
A disco robot which explodes into fish pastries when defeated.
A zombie dinosaur robot pushing a machine-gun-toting carriage with a large slime monster inside.
A brain-robot which warps around the room.

After defeating the third boss, the player gets sucked into a portal and warped to the Mesozoic Era and becomes the king of monkeys after several years pass. The credits then roll.

Contra: Hard Corps has significant differences between the Japanese, American, and European versions. In the Japanese version the characters all have a lifebar, and can be hit three times. This version also has unlimited continues unlike the American and European (PAL) version. The American and European version does not have a lifebar; a single bullet will take a life. This makes the American and European version significantly more challenging.

The European version also has significant censorship. The game is called Probotector and all the characters in the game are robots. Furthermore the plot and cut-scenes are different. The antagonists are aliens and the Alien Cell's name was changed to the "X-Drive".