Angel Island Zone

Angel Island zone is the first zone in Sonic 3. Playing as Sonic, you get to watch a rather lengthy intro. The Sonic 3 beta intro sprites of Sonic surfboarding can still be accessed from debug, apparantly by using the bonus stage glitch.

It's also interesting to note that the area where the miniboss sets fire to the island is covered by 128 tiles, rather than the backgrounds switching while the screen is covered in fire.

I have also found a glitch using debug mode where you go into Knuckles' path as Sonic, then go up to Sonic's path after going through the long set of breakable walls. When you get to the boss, there will be two of them - one acting as Sonic's boss and one acting as Knuckles'. This can be used to give a 50000 point time bonus if you use one of the capsules to trigger the end of level and reset the time to zero, then use the other to trigger another end of level.

When at the end of the level you can also hold left and repeatedly try to jump while Knuckles is doing the cutscene, to stay on the platform for as long as you like.