Perfect Playthrough: Sonic 3 & Knuckles Part 14 (Blue Sphere)
0If you lock on the original Sonic the Hedgehog on top of Sonic & Knuckles, you get a screen with the Sonic cast taunting you for inserting the wrong game. But if you press A+B+C all at the same time, you get access to a secret game called Blue Sphere. Since Perfect Playthrough conditions involve getting a perfect in all 134 million stages (yes, really), and the 24 hour speedrun only covers a bit more than 2 thousand of them, this is a perfect example of bad opportunity cost, so I'm gonna pass over the Perfect Playthrough conditions. However, I did decide to play a couple of these levels just to show off the game in its entirety (Sonic 1 has them all).
Outside of Sonic 1, you also gain access to specific Blue Sphere stages when locking on various Sega Genesis games. But of the 20 non-Sonic titles in my possession, only four of them were compatible with Sonic & Knuckles. The four games in particular were Altered Beast, Sol-Deace, Sports Talk Football '93 Starring Joe Montana, and Mortal Kombat.
EA games seem to be completely incompatible with Sonic & Knuckles--not even Road Rash. Outside of the dozen games in my possession, WWF Raw, World Series Baseball, Double Dragon V, and Tecmo Super Bowl are incompatible as well. You just get the Sonic & Knuckles title screen instead.