Enter these on the password screen for the desired effect. Grid order from Left-to-Right, top-to-bottom. The password grid will turn blank again if they're entered correctly (with exception to the Level Skip cheat). After entering any of these, either choose "BACK" to go back to the title screen and start the game normally, or enter a proper level password. Note: The "Limited Invincibility" cheat will make you invincible to most hazards except for spikes, bonfires, Pinky & Brain's "vacuum tuba" device in Stage 3, and "Instant Kill" situations.
Space, Yakko, Dot, Yakko, Dot, Dot, Yakko, Dot, Yakko - Limited Invincibility:
Space, Space, Dot, Dot, Dot, Wakko, Yakko, Yakko, Yakko - Unlimited Lives:
Space, Yakko, Dot, Wakko, Dot, Dot, Yakko, Space, Yakko - Unlimited Time:
Save your score
To save your score, lose all your lives and say no to the continue screen. After that, you will be given a password in which you can continue your game with however many stages you cleared and the score you had.
In the second stage after the chandeliers, there will be a crate. If you push it all the way to the right, then the graphics will start becoming blurry and you won't be able to progress past the boulder lift. The only way to get rid of it is to lose a life or turn off the game. The only harmful effect is losing a life to get rid of it.
Unlock Stage 4
To unlock Stage 4, beat stages 1-3. After the credits, do to the password screen and there will be a password. Press Start to get Stage 4.
Unlock Stage 4 - Beat Stages 1-3, enter password given to you
Complete Levels 1 and 3:
Enter the password Dot, Yako, Yako, Space, Yako, Dot, Dot, Dot, Wako.
Thanks to Revolution readers Martin Brodeur!