Sonic 4 EP1 Boss Theme was actually from an early prototype of Sonic 3D Blast
Sonic 3D Blast 825 Prototype
The Big Arms boss from Sonic Generations 3DS originaly used the Doomsday Zone theme instead of the Sonic 3&K Final Boss theme
The First Boss In Sonic CD is called the EGG-HVC-001.
The Last part HVC-001 is the Code name for the Famicon
Sonic's Top Speed animation from Sonic Mania is a reference to Sonic 2 Beta.
The Theme for Sunset Park Act 3 from Sonic Triple Trouble is a lower pitch version of an unused theme from Sonic Chaos