Oh no, retail game. =>.<=

For me it was a prototype.

https://hiddenpalace.org/Sonic_2_%26_Knuckles_(May_24,_1994_prototype)

Removed the Sonic 2 ROM and replaced with Sonic 3. Several differences. Gotta get savestates.

Sonic and Tails ending does not seem to be implemented, so after Death Egg act 3 boss is defeated, crash. The boss itself works, but some differences. Plays Death Egg act 2 music, some other diff.

Interestingly, Knuckles does have a working though incomplete ending. It doesn't crash.

Clearing Doomsday Zone doesn't crash, but no ending sequence either. Jumps to level select.