Some classes are known smokers: the Spy and the Soldier. This increases to 6 with some hats (to be exact, the classes are the Demoman, the Sniper, the Spy, the Soldier, the Medic, and most recently the Heavy). The Pyro carries a lighter, but probably for a differentreason.
When the Engineer is gibbed with the Gunslinger equipped, his right hand will still be the normal gloved hand.
Similarly, if a player wearing an unlockable hat is gibbed, the regular hat gib may still spawn.
Engineers can have both the Short Circuit, a replacement for the Engineer's arm equipped, and the Gunslinger, a replacement for the Engineer's arm equipped. Neither one fits into the other.
The Horseless Headless Horsemann's Head and the Saxton Hale Mask have an unspecified bonus. Specifically, they protect against the Horsemann's Scare taunt.
The Horseless Headless Horsemann doesn't do damage in the normal way. His attacks are treated as environmental kills, much like getting run-over by a train in other maps or falling down a pit. You can see this if you look at the console after he kills people. The coding for such a death, Horsemann's axe included, hits you for twice your current health; this prevents his attacks from being survivable in most normal ways.
According to his YouTube channel, Nathan Vetterlein (voice of the Scout) claims he's actually terrible at Scout, and prefers to play Heavy.
During one of his many Twitch streams, he mentioned that he reinacts a scene from DieHard to warm up for his Scout voice, basing the Boston-accent voice on John McClane.
The 'bullet' the Heavy holds up near the end of "Meet the Heavy" is a 7.62x39mm cartridge; the same kind of ammo used in the AK-47 which like the Heavy, also hails from the Soviet Union.
Many of the expansion items were suggested by fans. Before the Huntsman was released, there had been a 40+ page thread discussing a bow and arrow for the Sniper on the official forums. The most requested ones were the Heavy's Sandvich and the Scout's Bonk! Atomic Punch. The Spy's Dead Ringer (cloak and leave behind a corpse) was actually programmed in on some fan-altered servers before Valve implemented the idea into the regular game.
And for those that weren't implemented into the game, they live on in the Advanced Weaponiser mod.
Several of the hats, such as the Scout's Bonk Helmet and the Pyro's Brigade Helmet, were originally created by a group of highly talented modelers whom Valve took interest in. Although Valve had already planned to include similar hats in future updates, they liked them so much that they based their versions off the custom models.
The King of the Hill gameplay mode was originally a custom mod done by the No Heroes TF2 community before Valve implemented a slightly modified version as an official gameplay mode.
Similarly, the Payload mode is based on the fan-made map Happycow.
Valve has also opened a submission site for custom avatars and items.
As of the Über update, the fan terms "Demoknight" and "Pocket Medic" have been accepted into canon.
When fighting him as a boss, Merasmus will do a homage to the Prop Hunt mod and spawn random objects around the map while turning into one himself to hide amongst them at both 50% (one half) and 10% (one tenth) of his HP. Once he's discovered, he'll immediately turn back to normal and attack the player who revealed him.
The Danza: The Administrator is named for her voice actress. ("Helen" and "Ellen" sound almost identical in most dialects of English).
Freudian Excuse: She believes that everyone else in the world is a driveling idiot. Given that she grew up in Teufort, where a chemical leak has tainted the groundwater and given everyone else brain damage, we can hardly blame her.
Development Hell: The game itself took nine years. Also, the last four Meet The Team videos (Sniper, Spy, Medic and Pyro); the other five classes had been released relatively early on, but then it took 2 years to finish both Sniper and Spy, another 2 just to do Medic's, and another year for Pyro.
Extremely Lengthy Creation: It Valve nine years to make and were close to spending ten on it. The devs were working on Team Fortress 2 after they made Team Fortress Classic. Then they became part of Valve and started working on a Goldsource version, then constantly changed everything around until they released it in 2007.
Doing It for the Art: The amount of detail put into the games' artwork is amazing, down to things you might not even notice, like how every class has a unique set of teeth. Most notable are Sniper's pointy canines and Scout's buck teeth.
Fake Nationality: None of the characters is of the same nationality and region as their voice actor, except maybe the Pyro.
Chet Falizek, writer: We wanted to make it sound like what Americans in the '60s would have imagined these people sounded like, not what they actually sounded like.
Fan Nickname: More on the trope's page, and even more on the official wiki here.
A Demoman wielding his melee weapon and shield is referred to as a "Demoknight" or "Sword and Board". "Targelander" and "Targe Demo" are also considered acceptable parlance for one with the Chargin' Targe and Eyelander. Likewise, a Demoman with a frying pan, Bounty Hat, shutter shades, and targe is called a Demopan.
Bitter players have taken to calling the Pyro "W+ M1" (forward movement + attack) due to the (alleged) notion that the only thing you need to do to win as Pyro is charge forward.
In similar fashion, the Loch-and-Load has become the "Lucks-and-Lolz" to some for its similar direct-hit-only damage style.
A Soldier with the Frying Pan and Stainless Pot may be called a Kitchen Warrior.
A Pyro with the Gas Jockey's Gear set and Detonator is a Det Jockey, while one with that set and the Flare Gun is a Flare Jockey.
An Engineer with the Frontier Justice and Gunslinger is a Revengineer, named for his ability to deploy Mini-Sentries quickly, get some kills, then kill opponents with "revenge crits" after the sentry has been destroyed.
Before the Enforcer's nerf, Spies that used the Enforcer and the Dead Ringer were known as Dr. (Dead Ringer) Enforcer Spies, since the Dead Ringer completely negated the Enforcer's original weakness.
When the Spy-cicle was released, Spies that used the Dead Ringer, the Enforcer, and the Spy-cicle were known as Dr. Enforcicle Spies, since the Spy-cicle had incredible synergy with the Enforcer and the Dead Ringer. This too died down with the Enforcer's nerf.
Heavies that use Shotgun or Family Business as their main weapon rather than their miniguns are called "Fat Scouts".
God Never Said That: It's commonly believed that every member of 1850's BLU team is a notable historical figure, but makani clarifies that the Medic and Soldier are not intended to be Sigmund Freud and Stonewall Jackson.
Sniper: John Patrick Lowrie does Sniper and most of the random male characters in Half-Life 2. And he's married to Ellen McLain, who does the Announcer. Definitely Valve's go-to couple — one or the other of them has been in every game Valve's put out since HL2.
As of the fully community created Robot Boogaloo update Valve promises a lot more of this.
Promoted Fanboy: Tony Paloma (AKA Drunken F00l), creator of the immensely popular SourceOP plugin and the infamous idling program, as well as (unjust) crafter of a Golden Wrench. He ended up being VAC-banned. Fast-forward to present day: Tony is now working for the TF2 team, after being hired by Valve last summer.
Valve often features fan works on TF2's main blog as well.
Prop Recycling: Noticeable in the interior of some maps are a pair of extension cords that go from one wall and plug into an adjacent wall. Playing PropHunt in different maps allows the player to notice familiar-looking props. The trope is averted with promotional items (even items that appear in another Valve title such as Bill's Hat), as new models are created to fit the style of the game.
Notably, the 119th Update was supposed to be the 100th Update.
"It's been a busy afternoon here iterating, polishing and playtesting a blog post to commemorate Team Fortress' one hundredth update. As often happens, we emerged from thirty minutes of Valve Time, covered with bed sores and sporting full beards, to discover weeks had passed. While we were out, somebody here shipped 19 more updates originally scheduled for release in 2008."
Talking to Himself: The Heavy and the Demoman share the same voice actor, while the Spy and the Pyro share another. Merasmus and Bombnomicon also have the same voice actor too.
Another Halloween update dialed it up even further, with a three way argument between Blutarch, Redmond, and Zephian Mann all shouting at each other and all voiced by Nolan North.
The Other Darrin: In the short film "Expiration Date," because Engineer's usual voice actor Grant Goodeve was otherwise occupied, all of the Engineer's new lines are voiced by Nolan North.
The game itself underwent many, many, MANY different iterations before the development team settled on the current gameplay and visual design. (For example, it was originally pitched as a combination FPS/RTS with a realistic art style, with a class specializing in commanding the other units around somehow.)
There were a few items and weapons that were scrapped during development. There was a tranquilizer gun for the Spy and the Scout was originally given the Sniper's SMG. The Scout was also planned on getting a nail gun which would have behaved similarly to the Medic's syringe gun. There was an item for the Engineer called the "repair node" which was also scrapped.
Character design similarly had a lot of changes:
The Heavy once had a mullet.
The Medic, Engineer, and Spy wore their class badges on armbands.
The Scout's apparent age and toughness bounced all over the place.
The Demoman was just plain stereotypical Scottish (kilt and all).
The Soldier looked more like he actually fought in the army.
Another original idea was to have the game's art style looking similar to stop-motion animation or "claymation". This idea later led to the animation style the current game has.
There were a few more ideas for Meet the Medic which didn't make it in due to killing the pacing and/or it not really meshing with The Verse's general aesthetic (BLU Spy's still living severed head is a nod to one of the scrapped ideas). Finally, they settled on THE iconic image (a Medic ÜberCharging a Heavy) and built the story around that.
Meet the Sandvich had many scrapped lines. Some of which are:
Scout: Give me back my leg bone! *whack* Don't hit me with it!
Soldier: You cannot hurt me! Pain does not hurt! *crack* I stand corrected!
Scout: He's like a bear! He's like a big, shaved bear that hates people!
When thinking of what song in "Meet the Pyro" to play in Pyroland, one suggestion was Tiny Tim's "Living in the Sunlight, Loving in the Moonlight"—a song probably best known nowadays for its use in the SpongeBob SquarePants pilot.
Grenades in the style of Team Fortress Classic were almost implemented in the final version of Team Fortress 2, but were taken out, due to the imbalance they posed. The only remnants are the cosmetic items on Soldier's sash.
The Buffalo Steak Sandvich was originally going to be a cactus, specifically peyote. It was changed to steak due to being part of Hibernating Bear set (which includes the Big Chief, an Native American feathered war bonnet), since Valve sensed there would be Unfortunate Implications about the Heavy going on a killing spree while dressing as an Indian high on a hallucinogenic cactus.