Sonic's graphic on the title screen of Sonic the Hedgehog has a belly that is obscured by the logo's ribbon. This is a leftover of the animatic of Sonic's appearance on the title screen. On earlier, less accurate emulators, including Sega's own emulator made for Sega Smash Pack Volume 1 on Dreamcast, a common error would involve Sonic's belly hanging over the ribbon.

In Sonic the Hedgehog, there is an unused "PRESS START BUTTON" graphic. This would eventually be used in an exclusive revision of the game included in the Sega Top Ten compilation cartridge for Brazil and Asia. This version of the game is otherwise identical to the original release.

The file header (the game's name used as an introduction for its code) for Sonic the Hedgehog is supposed to have a line break in the middle, so it reads as:


However, this line break was done through spaces instead of an actual line break, so if one plays Sonic the Hedgehog on an emulator that shows the file header name (such as Kega Fusion), it will instead read:


[Note: periods added due to VGFacts automatically removing multiple rows of spaces.]

Connecting Sonic & Knuckles to the first Sonic the Hedgehog will not allow you to play as Knuckles in the latter game. This was not due to hardware limitations as is commonly believed, but rather because the game's development team felt that, while Sonic 2 was designed with Super Sonic's flight in mind and thus had verticality to its levels fit for Knuckles' climbing, Sonic 1 was not, and therefore simply was not fun to play as Knuckles in.

Hirokazu Yasuhara was going to go to America to establish a new development team but because of the Gulf war between Iran and Kuwait, his trip was delayed. Yuji Naka and Naoto Oshima noticed him being upset and decided to let him join their team temporarily until he leaves. He ended up staying for a year to design the levels for major games such as Sonic the Hedgehog.

In an early version of Sonic The Hedgehog shown in a pilot episode for the TV game-show "Nick Arcade", a rolling ball can be found towards the start of Green Hill Zone Act 1. This was removed from the game's final release.

In the original international release, a short cut taken to the finish in the Labyrinth Zone would only allow the player to collect a total of 49 rings, missing 1 ring from going to the special stage. However, in the Japanese version, a ring was added so that people could get to the special stage if they went this way.

The Japanese version of the game has more parallax scrolling layers. An example would be that the clouds in some of the zones move across the screen, while the international version the clouds are stationary. The Labyrinth zone also has an underwater wave effect in the Japanese version. The only zones that were not changed were the Starlight zone and the Final zone.

Within the game's data is a set of unused sprites showing Sonic shrinking or falling away from the screen.

In the game's code there exists a sprite of a Goggle monitor, and some matching goggles to go with it. There is also an unused sprite of Sonic holding his breath, meaning these were likely once an underwater power up.

Scrap Brain Zone was originally known as 'Clock Ork Zone'. It's believed the name was changed because it was planned to be called 'Clock Work Zone', but no 'W' existed in the game's zone title font.

The Japanese band "Dreams Come True" are credited with composing the game's entire soundtrack. To promote their first tour, the band had concert posters with Sonic the Hedgehog on them, and painted Sonic on the side of their tour bus.

The game's soundtrack was composed on an Atari computer.

In early screenshots of the Genesis game, a UFO could be spotted, but was not present in the final game. Yet when Sonic the Hedgehog was released in 2005 for mobile phones, the UFO sprite was found hidden away in the files, but still unused.

There are hidden embedded Japanese staff credits on the "Sonic Team Presents" screen. With the aid of a palette editor or cheat code, one can hide the title card and invert the colour of the background to make it visible.

According to series developer Yuji Naka, the iconic "SE-GA" jingle at the beginning of the game took up 1/8 of the game's cartridge space.

The iOS and Android apps were updated in March 2013 by Sonic community hackers Taxman and Stealth from the ground up, using the Retro Engine also used in the HD remake of Sonic CD (the engine is named after Sonic Retro, a prominent Sonic fansite). Besides featuring a constant 60fps, this remake has a ton of extras, such as the ability to play as Tails, Knuckles and Super Sonic, being able to collect the seventh Chaos Emerald (which allows the player to become the aforementioned Super Sonic), adding Elemental Shields from Sonic 3 & Knuckles, turning on and off the Spike Bug and the ground/air speed caps, and, last but not least, you can add prototype characters and sprites through debug mode, like Splats the Bunny (a Green Hill Zone enemy that never made it into the final game) and the UFOs in Marble Zone.

Sonic was going to have a human girlfriend named Madonna. This was to copy Mario's girlfriend at the time, Pauline. She was removed by the product manager, Madeline Schroeder. She wanted to remove Madonna as to "soften" Sonic for children and foreign countries, as she thought the concept was too "Japanese". The concept of a human girlfriend was later used for Sonic the Hedgehog (2006)'s Princess Elise.

Sonic was originally going to be called 'Mr. Needlemouse', a literal translation from the Japanese word for hedgehog.

In Act 1 of Green Hill Zone, it is possible to die by going too fast. At a certain point in the stage, the camera may not be able to keep up with you when you dip downhill, causing the game to think that Sonic has fallen into a bottomless pit.