RoboCop Versus The Terminator/Secrets

Developers' room: Invincibility (requires cheating)
Enable TurboCop cheat mode by pausing the game and pressing: (a sound effect will play as confirmation). Then go to the very left edge of the level, and jump as high as possible while holding. You will enter a room with a photo of the developers, and be granted invincibility for the rest of the game.

Developers' room: Extra lives
Reach the top of a multi-level building with two enemies and a gun pickup on the roof. Jump as far left as possible from the left edge, and you should land on an invisible platform. Jump left again to be bounced up to a final platform (this one visible). Press to enter a secret room with extra life pickups and a photo of the developers.

Level 2: Streets of Detroit
After dropping down at the end of a long railing you will end up inside a building; instead of continuing to the next railing (on the right), go left. You will be blocked by an obstacle, but you can jump over it — continue left until you reach the wall, and then press to enter the secret level.

Level 4: Toxic Farm
As you descend the ladder after jumping over the first few flaming barrels, you can jump through the side of the large pipe with "Toxic" printed on it. Once inside the pipe, press to enter the secret level.