Sonic 1 fact. This is probably the earliest known leftover in Sonic 1.
Some news footage of Sega R&D aired about four months before the Tokyo Toy Show in June 1990. Although brief and not immediately obvious, this video is the earliest known footage of Sonic the Hedgehog: It shows a portion of prototype collision source code on a computer and a brief snippet of blurry gameplay on an adjacent TV screen.
https://files.catbox.moe/dczvwp.gif
https://youtu.be/CRphv7x97uw
The few seconds of "game" footage make it clear that this is extremely early in development, with crudely basic placeholder graphics and a test level with slopes, thought to be for testing loop-de-loops (in Sonic the Hedgehog, loops work by swapping out chunks on the fly; parts of what look like the bottom of a loop and the top of a loop can be seen here). There is also a small rectangle, which may or may not be the player, as well as a glitchy sprite with two frames whose purpose is unknown. There is an orange/yellow bar on the left side of the screen.
https://tcrf.net/Proto:Sonic_the_Hedgehog_(Genesis)#Unused_Objects
Present in the available prototype are objects ID 02 and 03, both of which line up with the room seen in the video. These objects date back to the game's earliest development stages. Their coordinates are fixed, and what function they were intended to serve is unknown. Both objects share the same sprite mappings and tile ID base, and have similar code. Object 02 has a simple timer built into it to alternate between sprites every 16 frames. They also expect tile information from VRAM at 0x9E00, which is the exact same location that the unused debug HUD digits are loaded. They do not interact with the player object in any way, and likely never served a purpose in actual gameplay.
https://tcrf.net/Proto:Sonic_the_Hedgehog_(Genesis)#Unused_Objects