In the mission shown during the gameplay demo shown at E3 2012, Aiden is shown killing Joseph Demarco. The mission was changed prior to release, and instead killing Demarco, Spoiler:Aiden now has to kill a hacker, Defalt.
Contributed by ClaudX
The town Pawnee is based on the real life town of the same name. However, the game portrays Pawnee as being located right outside of Chicago, when it is actually located near central Illinois.
Contributed by ClaudX
While modding the PC version, a user by the name of "The Worse" found almost all of the shaders and graphical improvements present in the 2012 and 2013 trailers for the game. It's possible that the PC version's graphics were downgraded so that it would not make the PlayStation 4 and Xbox One versions look drastically inferior by comparison.
Contributed by Boyobmas
In some randomly generated vehicles, there is a chance that the keys will have a keychain of what appears to be a My Little Pony character dressed as Aiden.
Contributed by Psychospacecow
Inside Iraq's hideout are a number of arcade machines featuring the logo of Far Cry 3: Blood Dragon.
Contributed by Dazz
During one of the optional Privacy Invasions, the player can see an emotional discussion between a stage II cancer patient and a woman working two jobs. The conversation and characters themselves are a direct reference to the TV series Breaking Bad.
Contributed by Dazz
Graffiti can be found around the city that resembles the Slenderman.
Contributed by Boyobmas
Aisha Tyler (famous for her role of Lana Kane in the animated comedy Archer) makes a cameo appearance as herself in the role of a citizen NPC. She can be discovered while exploring Chicago, but can be easily found in Connely Square just outside of the Willis Tower in the mission "A Wrench in the Works". She is also the subject of a Privacy Invasion activity.
Contributed by totallytman
Every sign in the game reveals a famous meme or quote when hacked.
Contributed by Tuli0hWut
Inside Nicole Pearce's house is a Rayman Raving Rabbid toy that laughs when hacked.
Contributed by Psychospacecow
During one of the gameplay trailers. a man can be seen dancing in his apartment. His information states he is a video game store clerk and has legally changed his name to "Eddie Kenway". This suggests the man is a fan of Assassin's Creed IV: Black Flag, as Edward Kenway is the name of the main protagonist in that game.
Contributed by Boyobmas
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T-Bone is shown to own both Assassin's Creed II and Far Cry 3, two other Ubisoft titles.
Contributed by Boyobmas
One of the camera feeds that the player can hack into shows a father watching his son play a video game. The father is mocking how the assassin in the game talks to the people he kills. This is a reference to the Assassin's Creed series (also developed by Ubisoft), in which the protagonists talk to the people they kill before they die.
Contributed by Boyobmas
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Watch Dogs actually takes place in the same universe as the Assassin's Creed series. This is proven by an email found in Assassin's Creed IV: Black Flag sent to Abstergo by Blume Corporation.
Contributed by Mass Distraction