Concept art of what would eventually become Knuckles the Echidna depicts him potentially using a shovel for digging and fighting, as well as using goggles.
In IDW Sonic #14, Dr. Starline attempts to kill Sonic and Silver by duping them to enter a base he self-destructed. This quickly riles the rage of Eggman, revealing that he has a sense of respect for Sonic as a nemesis, saying that he can only accept beating him a certain way.
Knuckles' dad attempted to genetically modify himself to become stronger, but then saw Knuckles' unhatched egg and decided to microwave his unborn baby with Chaos Emerald radiation which resulted in a genetic mutation of Knuckles having sharp protruding knuckles from his hands.
EGGMAN FUCKS, GET OVER IT. FUCK MORE THAN HIM IF THAT BOTHERS YOU SO MUCH.
Amy Rose seems to be the only character in the main Sonic the Hedgehog cast who can handle a casual punch from Knuckles.
Scott Pilgrim's high school band "Sonic & Knuckles" is a very subtle reference to the 1994 Sega Genesis game Sonic & Knuckles.
Sonic the Hedgehog does not know what hedgehogs look like.
Whisper the Wolf's design began with an in-game custom character in Sonic Forces. From there, she would gradually evolve into her final look.
Tails is very discontent with the fact that Eggman has settled on making Tails Doll rather than a Metal Tails. Moreover, this suggests that he finds Metal Sonic and Knuckles to be particularly cool.
When robots or lackeys fail, Eggman uses his built-in command grab
In 2015, it was revealed that Ian Flynn, Tracey Yardley!, and Ben Bates of Archie Sonic the Hedgehog fame had pitched a line of Super Mario comics to Nintendo. It was ultimately rejected. Allegedly, Mario was also intended to cross over with Sonic and Mega Man in comic form.
Knuckles the Echidna's father dies of pancreatic cancer in the Archie Comics.