The primary source of inspiration for McCree's gameplay was the Colt Python revolver from Valve's Half-Life series. The developers loved "how the revolver felt" and wanted to make a character that would give players the same feeling.
Contributed by Kitsune Hawk
During the "Year of the Rooster" event, the main menu sometimes featured D. Va in her special event skin. The short piece of music which accompanied this was based on "Arirang", a Korean folk song.
Contributed by Kitsune Hawk
Hongyu Wu, a Chinese Overwatch player, lost his life whilst bravely chasing a motorbike thief on May 23rd, 2016.

A memorial was placed in his honor on the Control Center map of Lijiang Tower, where the name "宏宇" (Hongyu) is placed on one of red team's space suits. The phrase "英雄不朽" (Heroes never die) is also added to the background behind it.
Contributed by Psychospacecow
Genji's character may have been inspired by Hikaru Genji, the main character of an 11th century Japanese novel called The Tale of Genji. In addition to sharing a name, they both are described as playboys who were banished from their families.
Contributed by Mass Distraction
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During the game's development, Pharah (then only known as "Rocket Dude") was male and just "a dude in jetpack and a rocket launcher":

"For Pharah, I think we started with Jeff Goodman’s original design of how about a hero with a jet pack and a rocket launcher? That was it. The gameplay was solid, but we didn’t really have a good visual. It sounded like a very simple concept, but it really took us a lot of iterations to get to where Pharah is today.

[His name] used to be Rocket Dude, but we went back and forth. One of the first original concepts by Justin Thavirat... it was a super high concept (but I loved the shapes and the silhouettes); but then we looked at it and we were like... it’s a little too high concept and we wanted to ground it a little, so then you see the next one: “Oh yeah! It’s a dude in jetpack and a rocket launcher” and then suddenly we almost stripped down everything interesting about the hero. We went through several other iterations as you can see but retaining some of that original spirit of Justin’s original sketch and points from all these ideas from all these different artists we were able to come up with the final design which I’m super proud of."
Contributed by Mass Distraction
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During the game's closed beta, Bastion used to have a shield in front of him while in his turret form.
Contributed by Mass Distraction
D.Va is canonically a professional Starcraft II player, and she has a profile on the real life Starcraft II World Championship series website.

A number of signs and posters revolving around her sponsorships can be found on various maps.
Contributed by Psychospacecow
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In the Hollywood map, you can find the Warcraft Alliance lion logo accompanied by the text "GOLDSHIRE PICTURES" on one of the studio's buildings. This is a reference to Goldshire, a town located next to the Alliance capital city of Stormwind in the Warcraft universe.
Contributed by Psychospacecow
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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 II.
• "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 map. 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 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 was 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 Lúcio 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 "Rejuvenescência".
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