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The attacker spawn zone for the Hanamura stage is an aracde featuring several game cabinets containing references to other games:
•Fighters of the Storm, which features Tyrael from Diablo and Tychus Findlay from StarCraft 2.
•Fighters of the Storm 2, which features Sarah Kerrigan from Starcraft and Garrosh Hellscream from Warcraft.
•The Lost Vikings VI, an homage to the Blizzard game The Lost Vikings featuring a Starcaft 2 viking spaceship.
•Siege Mode 3, which depicts a marine and siege tank from the StarCraft universe and resembles the Metal Slug franchise.

The Fighters of the Storm arcade cabinets, which borrow characters from multiple Blizzard franchises, are named after the Blizzard game Heroes of the Storm, a MOBA game which also features a large cast of Blizzard characters.

The name of the arcade itself can be seen from outside of the building and is called 16-Bit Hero. It's mascot resembles a pixelated version of Jim Raynor from the StarCraft universe.
Contributed by Psychospacecow
In the Dorado stage, you can find pinatas styled to look like the Diablo character from the Blizzard game Diablo.
Contributed by Psychospacecow
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The mascot for Rikimaru noodle bar in the Hanamura stage is a murloc from the Warcraft series. The logo for the shop can also be seen on the sides of trucks and buildings throughout the game. Shooting at the statue of the murloc above the store plays the iconic murloc sound clip.
Contributed by Psychospacecow
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The Blizzard game Hearthstone can be seen on numerous tablets and computers found throughout several of the maps.
Contributed by Psychospacecow
NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
On May 5th, Overwatch entered public beta. Coinciding with this event, the pornography-hosting website Pornhub had reported that its searches for the game had increased by 817%. More than 600,000 searches for Overwatch related content on Pornhub had been made by May 23rd. Additionally, a website dedicated to Overwatch content by the name of OverwatchHentai.net has been created.

Given the nature of said content and the game itself being intended for a more broad audience, numerous takedown orders have been filed against creators of this 3rd party content, allegedly due to some creators using stolen assets taken from the game.
Contributed by Psychospacecow
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The "Arrivals" board in the Numbani airport terminal features the locations of Activision Blizzard offices around the world: Seoul, Cork, Buenos Aires, Madrid, Paris, Shanghai, Austin, Mexico City, Los Angeles, Versailles, and Irvine.
Contributed by DrakeVagabond
According to sound design supervisor Paul Lackey, the "hit-pip" sound played when you successfully shoot another player was created using the sound of a beer can opening.

According to Lackey:

"Another extremely challenging sounds was the 'hit-pip.' When you hit someone, you need to know you made contact. The sounds needs to cut through the mix but not feel like it comes from any hero. It went through tons of iteration. Finally, one night I thought, 'It should be satisfying to hit an enemy.' Just think about what's satisfying: beer. So I literally opened a beer can. Pssht. The sound is reversed and tweaked a little, but that sound is our hit-pip."
Contributed by Psychospacecow
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The development crew working on the previous Titan project (the precursor to Overwatch) once received a frustrating constraint from higher-ups that banned the use of flying cars in their game. As a cheeky response to this, Overwatch Director Jeffrey Kaplan decided that every car must fly in Overwatch.
Contributed by Mass Distraction
In the Temple of Anubis stage, the spawn zone vehicle features what appears to be artwork drawn on its hull of a Murloc from Warcraft with its signature "Mrglrglrlgl" noise written below. Inside the same vehicle, there is a trophy of a Hydralisk from the StarCraft series.
Contributed by DrakeVagabond
In the barracks of the Watchpoint: Gibraltar stage are lockers with the names of the game's Environment Art team: (Helder) Pinto, (Dion) Rogers, (Philip) Klevestav, (Andrew) Klimas, and (Bram) Eulaers.
Contributed by DrakeVagabond
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During early concept stages of the game, Hanzo and Genji were once originally the same character, a cyborg ninja named Hanzo that wielded both a sword and bow. As time went on, Hanzo was split into two characters, with Hanzo using the bow and arrows, and Genji who was given the cyborg ninja motif and sword.
Contributed by G-Haven
The two songs that LĂșcio plays during his Crossfade ability have been officially released by Blizzard onto Soundcloud under the album name "Synaesthesia Auditiva". The speed-boost track is called "We Move Together As One" and the healing track is called "RejuvenescĂȘncia".
Contributed by G-Haven
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The coffee shop in Eichenwalde lists "Kosakezipfel" on its menu. This is a reference to Loriot, a German comedian. "Kosakenzipfel" is a fictional dessert that originates from one of his skits and roughly translates to "Cossack's Cusp".
Contributed by Psychospacecow
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During Overwatch's beta, a pose for Tracer was removed after some beta testers deemed it to be too needlessly sexualized. The pose which replaced it is an homage to a work by Alberto Vargas, a pin-up artist.
Contributed by Psychospacecow
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D.Va's first design could be seen on a wall piece at one of Blizzard's events. Since every other character on the piece sports their final designs, this would suggest D.Va was one of the last characters from the original roster to be finalized.
Contributed by Mass Distraction
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NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
In the escort level Route 66 during the game's beta, there were magazines featuring the characters Symmetra and Mercy littered inside the outhouse, suggesting that members of the Deadlock Gang who reside there masturbate to them. The magazines were later removed in a beta patch.
Contributed by KnowledgeBase
In the Overwatch Cinematic Trailer, a security guard can be seen playing Hearthstone on a tablet device.
Contributed by DrakeVagabond
Overwatch was born from the remnants of Titan, a sci-fi MMO Blizzard had been working on for years but ended up cancelling.
Contributed by Mass Distraction