Sonic the Hedgehog (Genesis)

Sonic the Hedgehog is more or less the definitive game of the Genesis in the early 1990s and introduced the world to Sega's new mascot.

Level Select
One of the most well-known codes in gaming history: press Up, Down, Left, Right on the title screen, then hold A and press Start. In the original revision, the level order is different from the order in normal gameplay.



Debug Mode
Another well-known code. While Up, C, Down, C, Left, C, Right, C is the most common version, a shorter version is C, C, Up, Down, Left, Right. The main requirement for the code is pressing C twice before the final input of Right. After entering the code, press Start while holding A and continue holding A until Sonic appears in the level.

To activate Level Select and Debug Mode, simply enter the Level Select code, push any direction on the D-Pad after the ring sound, then enter the Debug Mode code.

From here, you have access to a variety of things:
 * Invincibility
 * Infinite Time
 * Debug Coordinates (in place of score)
 * Sprite Counter (in place of time)
 * Placement Mode: Press B during gameplay. From there...
 * Press A to move forward one object.
 * Hold A and press C to move backward one object.
 * Press C to place the object.
 * Pause Options: press A to go back to the title screen, B to enable slow motion, or C to enable frame advance.

Special Stage
These can be seen in normal gameplay by falling out of a Special Stage using Debug mode.



Labelled as Zones 1-6, possibly used in Special Stages to mark which one you were in.



1 Up icon for Special Stages. This was found in the manual, but doesn't appear in the game.



A W block. Used in Special Stages, still have no idea what it did.

Monitors
These are visible if you fly over the goal in debug mode, as the end-of-level art loading routine isn't executed and the goal art is written over the top of the art for the monitors.



A Robotnik monitor. Possibly would have hurt Sonic, like in Sonic 2 and 3 & Knuckles.



An image of Sonic's unused goggles, leading some to believe it was originally a powerup.



A static monitor, used as a default for invalid object IDs. Scrap Brain Zone contains a number of these (apparently a victim of changing object allocations - the subtypes work perfectly if these are changed to solidity objects) which emit various glitched tiles when broken.



This monitor also appears in the Sonic CD Prototype 510, essentially giving the player a speed boost and invincibility.

Sonic


Sonic gulping air. Likely used for the air bubbles found in Labyrinth Zone, and Scrap Brain Zone's third act.



Sonic holding his breath. Possibly used for when the player was close to drowning.



A sliding sprite. Similar to those used in Sonic CD.



Seen in earlier development pictures. Apparently a victory animation used after passing the goal.



Spin Dash-like sprites. There's a routine which would change Sonic's sprite to one of these, but it's unused.



Shrinking, maybe falling, sprites for Sonic. No idea how these would've worked.

Green Hill Zone


This checkered ball, seen in early screenshots, and part of the Green Hill Zone boss in the final. It would've, originally, been its own object, and crashed through walls that Sonic couldn't break through.



Two faces, possibly from an earlier point in development. Their true palette isn't in the game anymore. They are simply earlier versions of the Green Hill Zone decorations.



Unused tiles from the Green Hill Zone.

Marble Zone


A horizontally-oriented spiked thingy. A fully functional but completely unused object exists for these.

Spring Yard Zone


A cut sign from Spring Yard Zone.

Labyrinth/Scrap Brain Zone


A Burrobot, facing downwards.

Star Light Zone


Unused lights from Star Light Zone.



Also, unused machinery scenery from the same level.

Final Zone


The sprite on the left shows the legs of Robotnik's vehicle, which are impossible to see ingame.

Ending


A rosebud that acts like a bumper. This can be placed in the Special Stage and ending scene. The palette is lost.

Press Start Button


These tiles, which would appear on the title screen, don't show due to a programmer's goof.

Splats the Bunny


An unused Badnik intended for the Scrap Brain Zone. Notable, in that appeared in lots and lots of promotional material before meeting the ax. No idea if this is the intended palette or not.

Goggles


Unused goggles poses. Assumed to be related to the goggles monitor, but may have just been used to show that Sonic was underwater.



How it looks assembled.

Unused Eggman Graphics


No idea, possibly an unused or cut ending sequence.

Invisible Credits


Credits that are hidden on the Sonic Team Presents screen, and completely unseen in every region outside of Japan. If you have a Japanese Megadrive, or switch the region to Japanese, you can use the code C, C, C, C, C, C, Up, Down, Down, Down, Left, Right, which should make a ring noise, then when the demo starts, enter Down + A + B + C to turn the screen's background to white, making the text visible when the demo finishes.

The credits translate to:

Misplaced Enemies
In Green Hill Zone, there are multiple misplaced enemies such as this Crabmeat in Act 2 at coordinates 10D0 0370, under the section with the moving pillars and spikes, just before the checkpoint. They cannot be seen in gameplay, as they're always placed such that they instantly fall into the abyss and disappear - the Crabmeat is particularly amusing in that it's already below the level of the screen.

Revisional Differences
A revision (commonly known as REV01) was silently released, which is more common in the Asian market. Most compilations and re-releases include this altered version. Below are the changes:


 * The game's background deformation and screen drawing routines were given significant overhauls. As a result, the clouds in Green Hill Zone and on the title screen now scroll from right to left (common in most depictions of Green Hill in the series) and the water in Labyrinth Zone/Scrap Brain Zone Act 3 has been given a ripple effect, similar to that of Sonic CD.
 * The level select was rearranged. In the initial release, the Zones were listed in a pre-release order that doesn't represent the final game.
 * There are more changes if the game detects whether it is being played on a Japanese console. Previously, it disabled the Debug Mode but enabled the hidden developer credits - the revision provides new level select and Debug Mode codes as well.
 * The points system was slightly altered: extra lives are now gained every 50,000 points, and the final boss now awards 1,000 points in defeat. Previously extra lives were not acquired upon reaching any particular point total and the final boss awarded no points at all.
 * Minor programming changes of certain properties such as Caterkiller's nuances, lamppost / game over and drowning music exist.
 * Generally, the game code is tighter, more efficient, and removed unnecessary remnants like certain unused sprites.
 * Holding buttons during the attract mode and ending demo sequences will not cause Sonic to miss jumps.

Labyrinth Zone's Extra Ring
There is one extra ring in Labyrinth Zone Act 1 of Revision 01. After the player presses a button near the two rising platforms, a platform spawns above, something few people know about. It's possible to reach this platform with a well-timed jump, and if the player rides on it, it'll take them to an alternate path which may have an extra ring. With this extra ring, it is possible to collect 50 rings (and so access the special stage) while taking this shortcut.