I always wondered why, in Sonic Adventure 2, only Eggman had the Protective Armor Upgrade and Tails didn't. But then I realized that Eggman's Egg Walker is just an assault platform, while Tails' Cyclone was an alternate form for his plane, the Tornado 3. Of course Tails didn't put extra armor on the Cyclone - it would have been too heavy to fly!
In Sonic Adventure 2's "Dark Side" ending it's shown that unbeknownst to Eggman, the laser has overloaded as he tried to fire it with the power of all seven Chaos Emeralds, which is what the Cannon's power was source was intended to be — whilst it's possible Gerald Robotnik underestimated the power of the Emeralds, in the "Hero Side" ending, Sonic manages to prevent the ARK's cannon from firing, potentially causing the resulting overload that occurs in the "Dark Side" ending. This makes even more sense when one remembers that in the "Hero Side" ending, there is a moment when you see Eggman's hand reaching for the Chaos Emerald. When Amy and Tails look out the window, and we have a clear view of the entire room, Eggman is gone, presumably having taken the Emerald to the cannon's core. Cue "Dark Side" ending.
The Last Story ending has a subtle reason for why Shadow powers down from his Super form & Sonic doesn't. You start the preceding boss battle with Sonic, and each successful hit switches the other Hedgehog in, whilst the one who is not being used collects some rings to stay powered up, and it's a six hit boss. Whilst travelling past the boss leads to the characters switching out again, Shadow is meant to be the one to get the last hit.
Dialogue during the fight has both Sonic and Shadow noting that Shadow is having a far harder time maintaining a super form than Sonic. The reason why he doesn't appear to do so, is very simple — Special Effects Failure.
No wonder Shadow's having a hard time maintaining his Super form when compared to Sonic it's only the first time he's ever gone Super, whereas Sonic has had previous experiences of doing so, and is more experienced at it!
Let's also not forget that Shadow just had a one-on-one showdown with his own prototype, the Biolizard right before going Super. He may well have been exhausted compared to Sonic, who took a breather between getting to the Biolizard's chamber and going Super, and couldn't maintain his Super form as well.
Sonic Adventure 2 seems to have an Idiot Plot where people can't tell the difference between Sonic and Shadow, resulting in Sonic being framed when Shadow steals the Chaos Emerald. However, it becomes fridge brilliance when you consider that the organization that blamed Sonic was G.U.N., which may have been framing him in order to hide Shadow's existence.
Considering the amount of research that Gerald had on the ancient Echidnas in Sonic Battle, Shadow may have actually been based on the artwork in the Hidden Palace that depicts Super Sonic with Dr. Eggman. While he would've noticed his semblance to the image of Eggman, the mural does not clearly show them fighting, and could even just be interpreted as the man giving the hedgehog the Master Emerald. It's an intentional reference to the picture, but an unintentional reference to Sonic, who just happens to be the picture's intentional reference. Confusing.
Sonic Battle suggests that Shadow was based on the mural in the Master Emerald Shrine. That Gerald was basing his figure on a legend. But heckaroonies, what's to say that Gerald didn't ALSO believe that it was a prophecy, and he was trying to make something that would FULFILL a prophecy of the future? Sonic is clearly fighting what is a robot... and Gerald wants Shadow to resist GUN's efforts to make him into a weapon!
GUN's mistaking Shadow for Sonic makes a lot more sense when you realize that Sonic is the only super-fast Hedgehog in the world, and if you saw one you'd be pretty safe in assuming it was Sonic, even if he was a different color. Maybe GUN was in fact Genre Savvy and thought that Sonic had turned evil and decided to commemorate it by changing his color, a la Venom.
The level timers on Prison Island. In the dark story, following the Eggman vs. Tails battle, we have a cutscene where Rouge is instructed to set the timers on the bombs for 15 minutes. Her level then has a time limit of 5 minutes — followed by Shadow's next level having a timer of 10 minutes. It gets better. In the Hero Story, Tails and Amy proceed to bust out Sonic, who runs around like a madman through the harbor. Neither of these levels are timed, because the heroes don't know about the bomb. After both Metal Harbor and White Jungle, there's the first Sonic vs. Shadow fight. After this, Sonic learns about the bomb. The next level for Hero Story, Green Forest, has a timer of 8 minutes. Considering that Metal Harbor can be completed in around 2 minutes (which is actually the requirement of the 4th Mission), it can be assumed that, canonically, Shadow gets through White Jungle in the same amount of time (which could be possible - its 4th Mission requirement is 3 minutes), leaving Sonic with the 8-minute timer for Green Forest. - Dan Genesis
Sonic nearly dying in Sonic Adventure 2 wasn't an Ass Pull. A key thing to remember about the Chaos Emeralds is their power over time. Which also leads to Destiny. In other words, it's Sonic's destiny to save the world. This literally explains why he's in the right place at the right time to save the world. Destiny finds a path for him to follow. ~Hydronix
A lot of Chao resemble characters in the game. Most notable are the Shadow, Sonic, and NiGHTS Chao. However, there are three special Chao, known as the Chaos Chao. These chao are effectively immortal (no reincarnation, no death), and are quite difficult to obtain. The dark and hero Chao resemble a demon and angel, respectively, appropriately enough. However, the third bears a vivid resemblance to Chaos 0. This may seem odd, or unimportant... until you remember that Chaos is a mutated Chao. That means that the Chao doesn't resemble Chaos; Chaos resembles his previous Chao form! - Kuiper
In Sonic Adventure 2 and then eventually Shadow the Hedgehog, the reason why GUN does not fully trust Professor Gerald Robotnik in the past could be due to his name. Robotnik is East European in origin, suggesting that Gerald might have possibly been a Cold War defectee from a Soviet state. Thus GUN might have been suspicious of him, and their fears about his loyalty might have been erroneously confirmed when the boy who would become the future leader of GUN witnessed Black Doom making use of Gerald. -Ace Of Scarabs
Knowing that Knuckles takes very seriously his duty as the guardian of the Master Emerald and has been accostumed to isolation, it didn't seem fitting that he also was recognized for something so mercenary-sounding as treasure hunting, let alone the lyrics in his theme song in SA 2 "-Large pots of treasure make me feel much better". Then I realized that looking for treasures implies looking for ancient things, things from civilizations lost in time just like his own. When you see it as a way of bringing light to things so that they may not be forgotten and in turn gaining a bigger sense of historical identity, it definitely sounded as the opposite of, letīs say, Rouge.
I realized that Sonic Adventure 2 showed that not all of Station Square was destroyed in Sonic Adventure, or at least it was rebuilt, which is a relief. Rouge has a newspaper that says Tails was given a Chaos Emerald at Station Square city hall for saving it from the missile attack. This could not have happened until after Perfect Chaos attacked it, since between the missile attack and the Perfect Chaos attack all the Chaos Emeralds were with Knuckles or by Big's house. So there had to be a city hall somewhere, and even if Station Square was rebuilt, it would be kind of weird to reward Tails for saving the city if it was destroyed anyway, so some part of Station Square must have survived.
It has been proven through the example of Tails that it is possible to copy the wavelength of chaos energy and encapsulate it in a physical form independent of the original Chaos Emeralds. Now pay attention to the switches in the Cannon's Core levels, notice how they freeze time? Now what is Chaos Control able to do? That's right, stopping time. Those switches probably aren't a gameplay abstraction to make the level easier but a canon addition to the Space Colony Ark. This means that Professor Gerald Robotnik figured out how to create devices which could replicate the effects of chaos control, similar to Tails' fake Chaos Emerald.
Speaking of SA 2, the massacre on the ARK may have had a larger reaching impact than we thought. Eggman reveals that, as a child, he idolized his grandfather for trying to make the world a better place — hinting before he became a villain, Eggman likely wanted to improve humanity (and still may want to improve us). Given the events on the ARK, the young Eggman likely saw G.U.N as corrupt. There's also the fact his parents would be incredibly paranoid of the government for killing Gerald and Maria. This would lead to a skewed upbringing for Eggman, and add his likely contempt for G.U.N, and it's pretty likely one of the major motivators for Eggman's villainy originates from the ARK massacre. If so, everything Eggman has done in the games may never had happened if the GUN soldiers decided not to shut down the ARK. This means that, if it wasn't for them, the millions who've died as a result of Eggman screwing around forces man should not know and incalculable powerswould still be alive.Dammit, GUN!!!
Furthermore, in all the Adventure games, Shadow and 2006, Eggman is clearly attacking countries affiliated with GUN (the most logical explanation for Soleanna to have GUN tech and crates is that they're a GUN-aligned country). It looks like he really, really holds it against GUN for ruining his Grandfather.
Another detail of serious consideration is that Robotnik as a name is a real East European (Polish/Russian) name. Fifty Years Ago could well have been during the Cold War. This also means that the Professor Gerald was a victim of Cold War paranoia, and it is highly likely the rest of the Robotnik family suffered the repercussions of Gerald being tried and executed and having their family name stained. Suddenly Dr. Eggman's past becomes a lot more bleak, either being ostracised due to his East European "commie" heritage in the US-like GUN states, or sent back to live in disgrace in the Iron Curtain bloc.
This extends into Fridge Brilliance when you realise the early Eggman installations of Scrap Brain and his early traps have a cruder, lower-tech feel, suggesting that he started with leftover surplus Soviet tech before he really began fabricating his own original hardware.
Keep in mind that, even without Gerald, the world of fifty years ago was still more advanced than nowadays — they still had mechs, and an active enough space program to create the Space Colony ARK. It's reasonable to assume that this happened at least 20-odd years from now. If so, the Cold War in the Sonic universe lasted over 80 years.
Imagine if the Hero story was canon. If Sonic destroyed the Eclipse Cannon after defeating Shadow, how would the Black Arms be stopped in Shadow the Hedgehog? Without the Eclipse Cannon, the only thing Shadow could do would be to warp the comet back into orbit, only for it to come back again the next 50 years.