Sonic Adventure 3 was going to made several times throughout the Sonic series. The first attempt was Sonic Heroes. The reason for the name change was because Takashi Iizuka, the game's designer, said that they would only pull in the hardcore Sonic crowd if it was an Adventure game.
The next game to almost be SA3 was Sonic Unleashed. The game was originally an Adventure game with the title of Sonic World Adventure. But when they added stuff that strayed to far from the Adventure games (Werehog, mostly.) they decided to make it a standalone game, but kept the title of Sonic World Adventure for the JP release.
In Sonic Adventure 2, ads can be seen all throughout City Escape and in Mission Street. In the boss fight with Hot Shot if you look behind Sonic in the cutscene before the boss, you'll see an ad for Phantasy Star Online, another Dreamcast title. Look behind the Hot Shot, you'll see an ad for
Plant of the Chaos." A reference to not only Planet of the Apes, but to also Chaos, the antagonist from Sonic Adventure 1.
Shadow was meant to die at the end of Sonic Adventure 2, but due to him such a fan favorite, they brought him back for Sonic Heroes.
In Sonic Adventure 1, in Eggman's base you see Eggman not building Metal Sonic, but also Silver Sonic from Sonic 2, as Silver Sonic MKII, who has only appeared in the comics.
In Mega Man 9, Splash Woman the only boss to take double damage from the Mega Buster.
Also in Mega Man 9, during the final cutscene with Wily, you see the cutscene from the perspective of the last Robot Master you beat, and that's why you don't see them.
In Sonic Unleashed (HD versions), during the Stage Empire City, if look around, you find a sign labeled Pachacamac Parking, a reference to Tikal's bad from Sonic Adventure 1.
In Sonic Generations, it is revealed that Eggman studied for a teaching degree.
In the original concept of Mega Man Zero, Mega Man X was going to be the main villain, not Copy X.
Almost all of the original Mega man games have sub-titles in JP, but none ever made it to the states. Here they are as follows:
Mega Man 2: The Enigma of Dr. Wily
Mega Man 3: The Death of Dr. Wily
Mega Man 4: A Newfound Ambition
Mega Man 5: Blue's Trap
Mega Man 6: The Greatest Battle in History
Mega Man 7: Fated Confrontation
Mega Man 8: Metal Heroes
Mega Man 9: Revival of Ambition
Mega Man 10: Threat from Outer Space
Mega Man X8: Paradise Lost
Mega Man Xtreme: Cyber Mission
Mega Man Xtreme 2: Soul Eraser
In the SNES Test Cartridge, during the burn-in test on the cartridge, it plays a 16-bit remix of the title screen theme of Zelda 2. The song is the seventh test for the sound module.
In Dead Rising, there is a store called "Jill's Sandwiches" a reference to the imfamous line from the original Resident Evil.
Also in Dead Rising, you can find a sign for "Protoman: the Movie."
Doduo and Dodrio can both fly even though they both don't have wings (But Scizor can't, despite having wings. Still not over it.) This mostly due to the fact the JP name for fly is "Sky Jump" And their Pokedex entries state how Doduo is known for his jumping ability
even though in the Pokedex Tentacruel is stated to have 80 tentacles, we never see more than 10 on the sprite (or 14 in the artwork.)
According to Mega Man & Bass, Wily is 57.
The reason that "Bad Box Art" Mega Man is in Street Fighter X Tekken is because that when Yoshinoro Ono asked Inafune about putting Mega Man in he stated that it would be "boring" and that we've seen Mega Man in a fighting game, and to give him "something original." He then decided to use "Bad Box Art" Mega Man, and "Made it even wackier" with the story that he has aged and gained weight since the picture for the box of Mega Man 1 was taken 25 years ago. Inafune loved it, and that's what put him in the game.