Due to being Dummied Out, Sonic 2's Hidden Palace Zone is literally hidden in the code (albeit without most of the graphics).
In the iOS/Android remake, you find the Zone by falling in the place where the infamous spike pit resided. The last place you'd expect to locate it!
Metropolis Zone contains starfish-like Badniks that explode and spread shrapnel. However, when they explode, there's no animal left behind. Conclusion? The starfishes are obviously some sort of proximity/timed mine made to look like a Badnik. When they detect Sonic, they move in; a timer (5 or so seconds) triggers the explosion.
In Hidden Palace Zone (a level that was removed from the final game), one of the giant emeralds is covering a tube that you can roll in, leaving you to wonder why it's even there. Apparently, the emeralds were originally intended to be breakable objects, meaning you were actually supposed to access that.
The reason why Robotnik can seemingly outrun Sonic to his Death Egg Robot? Because Sonic lets him. He's too noble to kill Robotnik, so he just settles for destroying his machines and foiling his plans. And he won't confront Robotnik unless it's a fair fight.
Plus there are captive animals in the machines waiting to be rescued.
Alternatively, Robotnik has realized he must Run or Die, as he is unprotected and less than a meter away from the fastest thing alive, that put up with almost a dozen of his contraptions and an Evil Knockoff. Robotnik's ready for the last stand, and he is running faster than he ever has before out of pure fear (or determination).
Speaking of the Death Egg Zone. Why is it only a short level in this game and not a full zone like in Sonic 3 & Knuckles?note It was actually intended to be a full zone, but they ran out of time. Because here, you hitched a ride on Robotnik's spaceship, leading you directly into the hangar, whereas in the latter game, you had to ascend from its entrance at the very bottom.
While this does require some extensive traveling across both acts of the zone to get rings, it is interesting to note that it's possible to obtain all seven Chaos Emeralds in the first zone, conveniently titled Emerald Hill Zone.
Oil Ocean Zone's music has a very Arabian feel to it. Quite fitting since Arabia has one of the largest oil reserves in the world.
Robotnik saving Sonic in the 8-bit version actually does make sense. In the story of this game, Robotnik plans on using Tails as a bargaining chip for Sonic to hand over the Chaos Emeralds. Since the first Chaos Emerald is obtained a bit earlier, Robotnik was saving Sonic because he doesn't want the Chaos Emerald to fall into the lava.
In this game, you can see Tails moving awkwardly while piloting the Tornado. By contrast, Sonic is completely calm when he's the one piloting it. This makes sense, as the Tornado belonged to Sonic, so he had more experience with it. Even though Tails is a genius, he hadn't meet Sonic until shortly before this game began, so he was still new at it. By Sonic 3 & Knuckles, he's more experienced and comfortable with it, so he no longer moves awkwardly when piloting it.
The Super Sonic transformation was Sega's way of paying homage to Dragon Ball and one-upping Mario (who becomes Super Mario by getting a Super Mushroom).
A possible In-Universe explanation for why the Hidden Palace Zone isn't in the final version of the game: it was so well hidden that Sonic couldn't find it!
The fact that Hidden Palace in the iOS/Android port uses the Mystic Cave 2 player theme instead of the unused Track #10, seems odd at first given the latter is more associated with the zone, but it makes perfect sense given that you access the zone by falling into the place where Mystic Cave's infamous spike-pit resided, making Hidden Palace technically a part of Mystic Cave.
You can access Hidden Palace in the iOS/Android port by jumping into the spike-pit. Why was it so infamous? If you fell in as Super Sonic, you wouldn't die as Super Sonic is immune to spikes, meaning you'd have to wait for your rings to run out and your Super form to end before you could die. Hidden Palace is where Sonic was originally meant to unlock his Super form.
Dramatic effect. The ending wouldn't be that good if it ended with Sonic/Tails just landing on solid ground after a massive drop and just pose. Besides, they did jump off the airplane to pose mid-air.
After all, Sonic characters have a certain history with falling from great heights with next to no consequences. Except Shadow.
He wanted to make sure Sonic wouldn't fall in the ocean and drown.
This could swing one way or the other. While it is true that there's no fall damage, it's also possible to die if one falls into a bottomless pit, and there has to eventually be some ground to hit afterward, even if it's not seen in the game. Alternately, it is possible for Sonic and Tails to move so fast they die from a fall, meaning that theoretically, Sonic (or his animal friends) could move at such a velocity that they break reality, becoming integrated with the landscape as they die instantaneously.
...Which should all be further evidence that those cute lil critters are boned.
In regards to this, Sonic Classic Heroes seems to be the first romhack to both give parachutes to the turtles and make the chickens fly upon being released at that level.
Actually, you CAN fall in Sky Chase Zone by spindashing too fast, causing you to die. So the same should hold true for the freed animals.
Falling in the Wing Fortress Zone means death as well, so fall damage is clearly a problem from that height.
The Oil Ocean Zone is an environmental nightmare. The waters are so full of oil that you can bounce off of it, and the skies are orange. Though many of the other zones on Mobius look pristine, pollution does not stop at human-designated boundaries, and a site like this zone is bound to bleed into other places on Mobius.
