The hat "Deep cover operator" for the Spy and the Sniper is a reference to "Snake" from the Metal Gear series. The hat was also contributed via the Steam Workshop under the name "Fox-Tie Wall Camo", a reference to Metal Gear's FOXDIE.

The back of the Bombinomicon, visible on the Eyeaduct, Ghost Fort, and Helltower maps, has a description of the book, three one-line reviews, and a price tag that reads "discount $6.66".

There is an unused texture for the voodoo-cursed Spy, which features his face stained with blood. The file itself appears to be used in-game, but because the alpha mask is completely transparent, the skeleton face below it shows through instead.

During the Meet the Medic trailer, when Archimedes gets hit with a piece of the Heavy's heart, a figure standing in an observation room above the surgical area can be seen.

This is most likely Miss Pauling, the Administrator's assistant in the the Team Fortress 2 comics, who keeps an eye on the mercenaries.

While wearing the "Magical Mercenary" Hat, the Soldier says "Friendship is magic, Creampuffs" and the Scout says "Friendship is freaking magic". Along with the hat's design, both phrases are references to the show My Little Pony: Friendship is Magic.

There are unused sounds for the Rescue Ranger. The sound names imply that at one point during development, the weapon was able to move buildings and to be charged to fire a (probably) more powerful shot. These abilities were presumably removed for balancing reasons.

At the end of the "Meet the Soldier" trailer, there are only 8 heads on the fence. The only head that doesn't appear is the Scout's.

It isn't simply off the screen however, as the 8 heads are all part of a single model file, which shows that the partially obscured head in the video is that of the BLU Soldier.

A Golden Frying Pan weapon was added in The Two Cities update. However, unlike the 19 generic Australium weapons introduced at the same time, the Golden Frying Pan shares an unusual trait with previous rare items made of Australium: enemies killed with it will turn into a golden statue. Whenever earned in a Mann vs. Machine game, a global message is displayed that states the owner's name. Since the update, only a few have been found, with the first one selling on the Steam Marketplace for $5,500.

The Neon Annihilator's battery has a nod to the Facepunch forums written on it: the texture reads "def2fp", meaning Death to Facepunch. The BLU version was originally intended to say "props2fp", but was changed by Valve, possibly unintentionally.

The weapon was originally supposed to be a chinese lantern, but the idea was dropped at Valve's suggestion since it closely resembled an unused weapon called the "Baleful Beacon".

The texture for the community-created HazMat Headcase item has a hidden note in the corner, outside the area used by the model. The message reads "Aca ♡ Prim 4evr~", meaning "Sparkwire loves Primrose forever", both of whom are credited with the item's creation.

The "Meet the Medic" class video was originally going to follow the interview format that the previous Meet the Team videos had used.

After a lot of iteration, with different scenes being scrapped for different reasons, the team realized they were putting too much focus on the creation of the Medigun and not the Medic himself. Once they found the right idea, the video came together relatively quickly.

The voice lines "No otha' class' gonna do dat!", "If you order now, I'll throw in a second beatin', absolutely free!", and "Look at this: just caved in your skull, my Bat's still dry. No clumps of hair - nothin'." were added in The Scout Update to the game after fans noticed a resemblance between the Scout and TV pitchman Vince Offer. Offer is best known for being the pitch man for the ShamWow! and these voice lines are parodied from this commercial.

One of the Scout's voice responses for stunning an enemy with The Sandman is 'Ding dong!'. This is a reference to The Crazy Church Guy from Left 4 Dead, who says this while ringing the church bell. Both characters have the same voice actor, Nathan Vetterlein.

The Team Captain hat is based off M. Bison's hat from Street Fighter. The flavor text "Our lawyers say 'YES! YES!'" is a reference to a scene in the Street Fighter cartoon where M.Bison exclaims 'YES! 'YES!' after witnessing Guile's defeat, which developed into an internet meme.

The Two Cities update introduced two new maps for Mann vs. Machine: Rottenburg and Mannhattan.

Mannhattan introduced a new kind of robot called "GateBots" that attempt to gain control of spawning gates on the map instead of carrying the bomb. These robots can be identified by the yellow GateBot lights they wear. Due to a bug, it was temporarily possible for players to craft GateBot lights.

Valve replaced the crafted GateBot Lights with an item called "Glitched Circuit Board". According to the item's description, the lights self-destructed after falling into human hands.

The robots voices in Mann vs Machine are simply the standard, human class dialogue passed through a robotic filter.

Among these voice files are unused Demoman and Soldier lines, generally stating "DOMINATED!", followed by expressions of concern or friendship.

It's likely that the original, human versions of these lines were created for the WAR! Update, in which the BLU Soldier and RED Demoman were discovered to have a secret friendship, and the administrator pitted them against each other. The unused voice clips imply that the Soldier and Demoman were putting on an act for their bosses, pretending to no longer be friends. It's unknown why Valve chose not to include these lines.

A glitch in the Spy's model will cause him to resemble a crab when holding his Disguise Kit and looking up.

The act of doing this became known as "Spy Crabbing", a light-hearted way to grief matches as the players would do nothing but walk around in this crab-like state. This eventually led to other players encouraging people to not kill these players and save the "Endangered Spy Crab".

Spy Crabbing became so popular that Valve not only opted to not fix the glitch, but added a taunt for the Spy that has a random chance of occurring when taunting with the disguise kit out. Instead of flicking his cigarette as usual, the spy will squat down, raise his arms like a crab, and click his fingers and disguise kit together like pincers.

In December 2009, as part of the WAR! Update, Valve ran a competition between the Soldier and Demoman classes. The objective was to kill as many of the opposite class as possible, with reward going to the victor.

At the end of the 8 day event, it was revealed that the Soldier was the winner with over 100,000 more Demomen killed than Soldiers. As a result, the Gunboats were added to the game as a secondary item for the Soldier.

For years it was speculated what would have been added if the Demoman team won, until lead designer Robin Walker replied in an email: "It would have been the Gunboats. It was an item we'd tested with both, and seemed to work fine with either." -- Robin Walker

The idea for the Spy came from a bug which caused some enemy players to appear as allies.

The voice actor for the Spy also voices the Pyro, and the voice actor for the Demoman also voices the Heavy.