Secrets:

Did you know that a Blue Knuckles existed? If you look closely, the sign at the end of the level of Sonic 3 depicts Blue Knuckles. The icon next to the lives is Sonic. To get the character, in the level select, you need to force the game to give you the character number 4.

Did you know that the first Sonic 1 prototype had a giant ball next to the first ramp and pink flowers? If you push the ball to the ramp, it will go against you.

Did you know that there are atleast 10 prototypes of just Sonic 2?

Did you know that you can jump at the end of an act in the Sonic 1 prototype? It used the exact sprite that is here.

Did you know that the first Sonic 3 intro image was this one? It also used a different knuckles sprite and the Sonic 2 sprites with a different shading.

Did you know that if you input: up, down, down, left, right then B, you can access the level select menu in Sonic CD?

Did you know that a Segasonic popcorn shop game existed?

Did you know that if you hold the button C on your controller at the start of the Sonic 1 demo you take Sonic's ability to jump?

Did you know that there was a Sonic CD Lite edition on mobile?

Did you know that the code for the level select of Sonic 1 is: up, down, left, right then hold A and press start?

Did you know that the first game that featured Tails was Sonic 2 from the Master System and Game Gear?

Did you know that there is a really bad ending for Sonic 1? If you dont collect all the Chaos Emeralds and dont hit Eggman at the end of the last stage, Sonic will run backwards, look at you with a dissapointed face and then jump?

Did you know that the code for the debug mode for Sonic 1 is: up C, down C, left C, right C, then hold A and press Start?

Did you know that the first zone took 6 months to make in Sonic 1?

Did you know that Marble Zone from Sonic 1 used to have UFOs?

Did you know that Spring Yard Zone was called Sparkling Zone?

Did you know that Scrap Brain zone was called Clock Ork Zone?

Did you know that before Sonic there was another hedgehog named Mr.Needlemouse?

Did you know that the second level of Sonic CD was cut from the game? The code name for it was R2. It was named this way because the PC version stored the levels in codes (R1, R2, etc.).

Did you know that to access the sound menu in Sonic CD you have to press: down, down, down, left, right then A?

Did you know that if you put 40, 12, 11 in the sound menu, you can access this image?
It's also how you activate debug mode.

Did you know that if you put 44, 11, 9 in the sound menu, you can access this image? The word spelled is "you are cool".

Did you know that if you access Tails in the 2011 re-release of Sonic CD, you will get this image?

Did you know that if you put 42, 4, 21 in the sound menu, you can access this image? This is a Batman tribute.

Did you know that if you put 46, 12, 25 in the sound menu, you can access this image? The translation is: "Infinite fun, SEGA Enterprises.".

Did you know that if you put 7, 7, 7 in the sound menu in Sonic CD, you will get access to a secret special stage?

Did you know that if you put 42, 3, 1 in the sound menu, you can access this image? This is the coolest image ever.

Did you know that Sonic CD was called CD Sonic?

Did you know that if you enter right, right, right, up, down then C you will see the game designers's best times for the time attack mode?

Did you know that if you dont move for 3 minutes in Sonic CD, Sonic will say "I'm outta here!", jump off the screen and lose all of his lives?

Did you know that to unlock the level select in Sonic 2, you have to play sounds 19, 65, 09, 17 and hold A and press start on the title screen?

Did you know that to unlock debug mode in Sonic 2, you have to play the sounds in the level select screen 1, 9, 9, 2, 1, 1, 2, 4?

Did you know that the first prototype of Sonic 2 was the Nick Arcade prototype? If Tails got hit, you would lose rings and if you ran into a wall, you would bounce off it.
(Image is from an autodemo of Sonic 2).
You should download it since I forgot to include it in the prototype roms.

Did you know that Sonic 2 was built off Sonic 1?

Did you know that Sonic the Fighters exists?
It was featured in the Sonic Jam collection.

Did you know that the Nick Arcade prototype only had two levels that had all assets working, Hidden Palace and Green Hill Zone?

Did you know that if you spam the waterfall item in Sonic 2 in debug mode and move your camera back and forth, you will unlock a special skin named Ashura?

Did you know that if you input the sounds 1, 1, 2, 4 in Sonic 2, you will only hear sound 4 for the whole period you play?

Did you know that if you press up, up, up, down, down, down and then up, you will hear a ring chime and then if you play as Tails, he will be called Miles?

Did you know that if you stay on one of those swinging objects in the first zone of Sonic 3 and press left, left, left, right, right, right, up, up, up, you will unlock the sound test?

Did you know that if you stay on one of those metal pullies in Mushroom Valley in Sonic 3 and press left, left, left, right, right, right, up, up, up, you will unlock the debug mode?

Did you know that at the start of the Sonic 3 prototype, theres Sonic surfing?

Did you know that there were monochrome sprites of all the characters in Sonic 3 dying?

Did you know that McDonalds actually made Sonic toys?

Did you know that the Sonic 2 Nick Arcade prototype was sold for 10.000$?

Did you know that to unlock the debug mode in the first Sonic 3 prototype you can use the Sonic 2 code or input the sounds 1, 3, 5 and 7?

Did you know that in Sonic 3 your ability was originally the drop-dash? Christian Whitehead said it's a pure coincidence that it was added to Sonic Mania.

Did you know that in the first Sonic 3 prototype there was an ability that let you shoot rings at badniks? (no image for that)