This is Real Life. Everyone knows zombies are make-believe creatures, they're only "real" in the movies. If you see a "zombie", odds are it's one of your friends with poor taste who insist on scaring you, or someone who has been in an accident of some kind. So when a zombie outbreak starts, it's unsurprising that most people will chafe when confronted with something that — well, only exists "in fiction". Still, you'd think most people have the presence of mind to back away from a smelly, shambling corpse of a man who is missing limbs and moaning for braaaains!...
That is rarely the case here. There will always be one person who misidentifies the living-impaired as a regular, if badly hurt, human. There's two variants of this, with differing levels of idiocy involved.
Someone is walking through the deserted area, when they see a moaning, pale-looking body lying face down on the floor. Said body may already be bleeding or even missing large, fairly visible chunks of flesh, and may already be ghost white or putrefying green, but the character in question will never notice (or ignore it). Instead, they will run right up to the body, put aside whatever weapon they're carrying, and promptly try to hoist up the apparently distressed individual (often accompanied by dialogue like, "Are you alright?" or "Don't worry, I'll get you out of here."). Hilarity Ensues when the putrefying face of the obviously-a-zombie is revealed, and promptly tries to eat the character's head. This is generally (well, hopefully) the first zombie that the character encounters, so they tend to survive.
Someone (usually a law enforcement official), points a gun at a zombie and orders them to freeze, or threatens them with pepper spray or arrest or something if they come towards them. They usually fail to notice the fairly obvious fact that the shambling mess of entrails in front of them doesn't have anything like a rational mind left to respond to commands, and assume they're not threatened by the gun (technically true). In general, this is a situation somewhat peculiar to zombie movies involving both Genre Blindness and a lack of common sense, when a character simply doesn't notice extremely obvious signs of something being deeply amiss with the people they are dealing with, up to and including, shambling, moaning, cannibalism and rotten flesh.
Depending on the officer/character's learning curve, they might fire a warning shot, notice no effect, shoot the zombie, notice no effect, then get suitably freaked out and do one of three things: hightail it out of there, use "lethal" force, or get eaten by refusing to believe it's a zombie. In real life, officers are rather constrained in this scenario by being trained to use "escalating force" when facing an "unarmed civilian". Bear in mind that even if they do go for the "lethal force" option, they're extremely unlikely to go for the necessary head shot — aiming for the center of mass stops most humans just as well, and the risk of missing (potentially hitting a bystander) is greatly reduced. A shot to the center of mass will generally not stop a zombie, unless the officer in mind plans on using extreme cases of overkill and restocking on ammo.
Rule of Perception is strongly tied to the trope, since the horrific stench you would expect a reanimated corpse to give off almost never comes into play, even in situations where the smell should by all rights be quite overpowering.
This is often easily survivable with Slow Zombies, as you've got a minute or two to realize something's up. Fast Zombies are not so forgiving, and probably only main characters will get away with this.
Generally, any character who thinks a zombie is Not a Zombie is also prone to having Zombie Infecteebehaviour or becoming one. If he doesn't accept that the creatures outside are zombies, he can't very well become one after being bitten, now, can he?
Contrast: Most Definitely Not a Villain, Paper-Thin Disguise. Not to be confused with Not Using the Z Word, where everyone acknowledges that the zombies are undead, they just don't want to use the usual term for them; and Totally Not a Werewolf, where a non-human of one type (let's say a Jiang Shi) gets mistaken for another similar one (a zombie) rather than for a living human.
Resident Evil Degeneration has both examples. Angela and the security guard in the airport at the beginning of the movie all try ordering the shambling corpses away from them to no avail. Before her stint at the "inefficient force" approach, the fantastically dimwitted Angela also slings a gray, moaning and clearly undead man over her shoulder to carry him to safety— not ten minutes after Leon has told her about zombies. Greg doesn't bother with the "warn the zombies not to approach before shooting them with nonlethal force" bit, blasting them square in mid-torso with his machine gun without the slightest hesitation, but ignores Leon's instructions to aim for the head, so all he succeeds in doing is wasting ammunition. Degeneration also plays with the trope by prefacing it with an instance where the aggressor is just a man in a zombie mask. This does give the victim a small bit of credit when it turns out that the next one is the real deal.
Films — Live-Action
Happens near the beginning of 28 Days Later, when Jim wanders around an eerily deserted London, then enters a church and finds a priest who is... a bit ill. Our hero doesn't recognize this at first, but then he has been in a coma since before the sudden outbreak. It helps that these aren't the slow rotting corpses of traditional zombie movies, but instead living, breathing, fast humans whose behavior has been modified by a literal Hate Plague.
The first zombie we see in Night of the Living Dead is supposed to look like just some random person wandering around the cemetery, until he attacks the girl. Doesn't help that in one colorized version, he's green.
When Shaun and Ed are walking home from the pub they see a zombie eating someone (they think they're a couple making out). Another zombie follows them but they mistake its moans as an attempt to join in with their drunken singing.
The following morning when (a hungover) Shaun walks to buy a paper and encounters several zombified people he usually interacts with (not noticing some blood and pushing a zombie tramp over when he gives him some change).
And Pete just thought he was being mugged by crackheads.
Finally they encounter a female zombie in their garden, who they at first think is drunk. They realize what's going on when she gets impaled on a pipe and gets back up.
There is also a comic strip which was created for publicity, which reveals the female zombie was infected when she tried to help another zombie up.
A scene added to the Doom movie's DVD release had two marines come across a naked female — who promptly turned around, revealing herself to be a zombie.
Subverted in Wasting Away were the protagonists are not aware that they are zombies, likely because they still perceive themselves as normal human beings with only the 'sober' uninfected seeing their true undead nature.
Averted in Plan 9 from Outer Space. Characters can somehow identify the zombies immediately, even though they don't look all that zombie-ish.
Averted in Return of the Living Dead. The first group to encounter a zombie knew about the chemical, and the first animated corpse they encountered was one they already knew to be dead. The second group encounters a zombie so horrifically rotted, and screaming for brains, that there isn't much question.
The Silent Hill movie contains an odd example. Cybil spends way too long waving her gun at a zombie in an effort to convince it to lie down on the ground. Keep in mind that it's a Silent Hill monster we're talking about here — a violently twitching, shambling monstrosity with a straitjacket of skin and a giant vertical slit for a mouth.
In the original 1978 Dawn of the Dead, a woman encounters her dead husband while a SWAT raid is taking place. Believing he's still the man she'd fallen in love with, she rushes into his arms... and gets a healthy chunk bitten out of her neck and shoulder by the briefly confused zombie; they are possibly the most convincing looking zombie bites in the history of zombie movies post-Night of the Living Dead.
Columbus has a flashback to his first encounter with a zombie. It was quite distressing for him, since it was his attractive female neighbor whom he had a crush on, and he nearly got bitten due to his reluctance to attack her, and the comical things he tried to use as weapons (such as a bag of cotton balls).
Inverted later when Bill Murray playing himself attempts to scare Columbus at Tallahassee's urging by pretending to be a zombie. Columbus immediately jumps up and shoots him.
In The Zombie Haiku Book, the writer notices one of his coworkers eating spaghetti in her car. The poem after that is him realizing A) that's not spaghetti, and B) there's something very, very, wrong with his coworker.
In World War Z, "The Great Panic", in which the growing zombie pandemic overwhelms the ability of governments worldwide to cope with it and results in the deaths of two-thirds of the human race, occurs in part because few people were able to cope with the idea that the walking dead were just that. The word "zombie" is also extremely rarely used, even once it's clear that's what they're up against. Slang terms like "Zack" or "Zed Head" are used, and occasionally "the living dead", but never "zombie".
This was actually a major plot point in Obsidian Butterfly, the last Anita Blake: Vampire Hunter novel: the apparently comatose skinning victims in the hospital were actually all inert zombies. Although in this case, it wasn't because people didn't believe in zombies, it was just that these zombies had been so well made that they still had all their vital signs.
Pride and Prejudice and Zombies has this happen to Charlotte. Elizabeth is the only one who notices this, and that's only because she already knew. This is despite several fairly revolting scenes such as the dinner at Rosings, where Charlotte eats her own bloody pus.
In Galaxy of Fear: City of the Dead, Zak's new friend Kairn is killed and then quickly made into a zombie. Zak has been dreaming of and was menaced by some decayed specimens, and thinks Kairn is different — and he is, he recognizes Zak, speaks to him, doesn't stagger, and smiles. Even though his friend is slowed and dulled, twitches at intervals, is sallow-skinned and unhealthy looking, and oh yes admits to Zak that he is dead, Zak doesn't really believe him and is surprised when Kairn leads him into a trap.
In Community, when military "taco meat" turns party guests into zombies en masse, this trope is played straight, as characters assume the zombies to be simply people on some kind of drug — until Troy averts it with "Holy crap, Leonard's a zombie!". The Dean threatens Troy with pepper spray later, thinking he is a zombie.
In Dead Set, a group of people stop and pick up a person who is being attacked by a zombie. Of course, nobody seems to care that a human being was eating him and are totally shocked when he starts biting them.
Lampshaded humorously on Supernatural in the episode "Dead Men Don't Wear Plaid", in which part of a small town cemetery (namely the most recent) are brought back from the dead as seemingly normal humans. Naturally, however, the brothers are suspicious and while visiting one of the first to pop back up, Sam (alone), finds the said woman alone in the house, laying on a bed, pasty green, and whispering, motioning for him to come closer. Sam, genre savvy, sighs and moves slightly closer. She continues to motion for him to come closer and he asks sarcastically if she can just "tell him from over there." Well, she continues to motion for him to come closer and he gets right up next to her, and sure enough, she wants him to put his ear by her mouth, whispering hoarsely. He sighs and says aloud "I know I am going to regret this," and leans in, and shizzam! She goes full on apeshit zombie on him and tries to eat him. However, he was expecting all this and shoots her in the head. Oh, and during the struggle he lands next to the badly mutilated corpse of her husband.
In the intro for the pilot episode, while looking beneath a car Rick sees a little girl's feet shuffling along. However, once he sees her fully after standing up and she turns to his voice, he realizes she's a zombie, and promptly blows her head off.
Inverted later on in the episode, where Rick is initially mistaken for a zombie and gets whacked in the head with a shovel. A few moments later, it's noted that as Rick was talking before being knocked unconscious, he's unlikely to be a walker, as they don't talk.
Castle did a zombie episode. Turns out, the murder was not committed by a zombie, but by a member of NY's zombie subculture, which Castle was upset to learn existed and he didn't even know about it!
Red Dead Redemption: Undead Nightmare plays the "law enforcement" part straight except, of course, John Marston isn't a lawman. John first calls the zombie on its unruly behavior before incapacitating it with a tap on the head. He then grabs his gun and starts shouting commands to stop. When gameplay resumes the objective is to blow the thing's head off. Justified to a terrifying level in that no one at the time the game is set in would have even a vague concept of what a Romero style zombie is.
Resident Evil has played the second type throughout the series. However it's averted in the first game when the Player Character realizes that the zombie isn't human (the fact it's encountered killing and eating a former colleague helps, of course).
It's a big subversion in Resident Evil 4 when Leon (who knows all about zombie outbreaks) takes down his first enemy and realizes, despite not appearing or acting like a normal human, "It's not a zombie!"
Resident Evil 5: Chris Redfield, you IDIOT. Sheva presumably has an excuse, but Chris does the whole type-1 thing (possibly excused by the fact that hosts of Las Plagas look and behave nothing like a traditional zombie, and Chris had never dealt with them face-to-face prior to this).
Something like this happens when the player first runs into a Despoiled in Wolfenstein. BJ blows up the Nazis' Black Sun machine, causing it to release a shockwave that vaporizes everyone in the room except himself and one particular Gestapo officer. The guy stands in place for a few moments, writhing in apparent pain, and BJ walks right up to him, presumably to force him to surrender. Imagine his (and the player's) surprise when the Nazi suddenly turns around and roars, revealing his skeletal face.
If you didn't know there were Zombies in STALKER: Shadow of Chernobyl, then the first time some loner comes stumbling towards you aimlessly, you might just pass it off as some random drunk Stalker. It doesn't help they can still fire their weapons. Plus, if you were unaware of their presence in the game, you'll be very, very surprised when you run into a random Stalker that can absorb half a magazine to the torso without flinching. Sure, A-Team Firing is in effect, but it's still unnerving when an enemy you've been able to knock off like popcorn suddenly refuses to go down and you have no idea why.
When Mark first encounters the zombies in Real Life in Weregeek, he doesn't believe in them, assuming they are a follow-up RPG to the vampire coven to which he's been just introduced. He is partly right: these are not real zombies but they also have nothing to do with role-playing.
Doctor: Well, your temperature is only 55 degrees, you have no pulse, no heartbeat, and your eyes are all puffy and sticky... Mortician: Oh no! You mean...? Doctor: Yeah, I'm afraid the two of you have pinkeye.
The episode "Marjorine" subverts this — Butters really isn't a zombie, but his parents won't believe it.
The Simpsons: "Homer, you shot the zombie Flanders!" "He was a zombie?"
In Scooby Doo On Zombie Island, Fred at first assumes that the zombies are just people in costumes and rattles off potential suspects as he tries to pull off a captured zombie's "mask". In his defense, this was the case for every one of the dozens of other supernatural creatures he and the other Scooby-Doo characters have encountered over the years, so it was a reasonable assumption. However, once the zombie's head comes off in his hands, he insists that it must be animatronic and Velma and Daphne tell him he's in denial.
In Gravity Falls, despite his slow gait, his slurred speech, his odd skin color, the "jam" on his face, and other details, when Dipper has evidence that zombies may exist in Gravity Falls, Mabel writes off the possibility of "NormalMan"- er, Norman being a zombie. She's right. He's a bunch of gnomes standing on each other's shoulders in a bad costume.