A common situation in a TV show with limited time to show events, where a character will suddenly appear in another character's home with no explanation. It can be interpreted as merely a shorthand to indicate that the character has in fact knocked and been let in but the scene is too boring to air. However, the trope often appears in contexts where that should not be possible, like when the person entering the house walks in on someone's private affairs, or is a villain, or is out of favor with the resident and walks in just to berate him, etc. Why nobody ever, at the very least, locks their door when they want to be alone is a great mystery.
Not to be confused with There Was a Door.
The Drop-In Character does this all the time.
See also: Swiss Cheese Security, Trespassing to Talk and Interrupted Intimacy.
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Happens all the time in Kannagi. The protagonist stopped locking his door after Nagi got to live with him (instead of, you know, giving her spare keys), and didn't lock the door when they both were inside as well. If people are ringing the bell at all, it is just that they are polite.
Mahou Sensei Negima!: Negi wanders into Chisame's apartment while she's dressed as her alter-ego "Chiu-chan".
Pokémon had an episode where Professor Oak suddenly appears in the "new character of the week"'s family business (a greenhouse) to provide plot-important information; no one bats an eye.
Door locks are nonexistent in To Love-Ru. None. Zilch. Nada. Not on bedroom doors, not on the school roof, and definitely not on the washrooms, which results in many Accidental Pervert situations in the name of Fanservice and Rito getting the crap kicked out of him by pissed-off girls.
In Fairy Tail, every guild member walks into Lucy's home as if they owned it, especially when she's sleeping or taking a shower/bath, sometimes together with her. Hilarity Ensues because Lucy is not amused.
Subverted by Rorschach in Watchmen as it is obvious that he broke the door and then waited for the owner to come home. Rorschach then criticizes the owner for installing such a cheap, flimsy Gordian Knot-brand lock.
In one Mai Hime Shizuru/Natsuki fan comic, Shizuru lets herself into Natsuki's unlocked room while she's asleep, and Natsuki finds her sitting across her table from her when she wakes up. Shizuru chides her for leaving her door unlocked, apparently from being tired from all the make-up work she's been doing, and Natsuki promises to be more careful in the future, but notes that no one out there is more "dangerous" than Shizuru.
The Katawa Shoujo fic From Shizune's Perspective has an example in which the intruder is also the one who forgot to lock her door. Shizune walks into her own unlocked room to find Misha and Hisao having sex with each other; Shizune had no idea that they were in a relationship at all.
Jake: There's a giant hole in the side of your house.
In The Jetsons movie,Furbie's parents do this after she follows Elroy home.
The Outsiders: Ponyboy explains that he and his brothers always leave their door open since you never know when a friend might need a place to crash, and they are rarely surprised to wake up and find some random gang member sleeping on their couch, or even a rival gang leader reading a newspaper while sitting in their chair, having already made breakfast.
Ponyboy: *Wakes up, walks downstairs and sees Tim Shepard on the couch reading a newspaper. He obviously came in while everyone was asleep.* ...Hi, Tim.
Tim: *Without looking up.* Hi.
Ponyboy: Can I..get you some breakfast?
Tim: Nope. *Folds up newspaper and walks out of house.*
It was lampshaded a few times in Friends too: when Monica and Chandler agree to move in together, Monica tells him to lock the door to her apartment, then unlock it again for the first time as her roommate. Chandler replies: "The door hasn't been locked in five years, but sure." Then in the final episode, Monica and Chandler leave their keys on the counter as they leave the apartment... and then so do all the others.
Kramer from Seinfeld and Charley from Empty Nest were both based almost entirely off of this trope— both neighbors to the main characters, both enter without knocking in a distinctive manner, and both steal food from the main character's fridge.
Although on Seinfeld it was averted with every other character, Jerry has to buzz George and Elaine into the building every time they come over whereas Kramer lives across the hall from Jerry.
Subverted in one episode. Kramer's drop in style involved sliding through the doorway as he opened it in a practiced motion. In one episode, Jerry actually locks his door causing Kramer to slam into it.
Lampshaded on Married... with Children when Marcy walked in through the Bundy's door and said, "Why do you keep your door unlocked. Any crazed killer could come in and murder you?" Al's reply? "Yeah, like I ever get what I want."
The show It Takes a Thief (2005) has two ex-cons who, with the owner's permission, and while videotaped, try to steal (then later return) things from the owner's house. They often gain access through unlocked doors. Then they help the owners improve their home security and try again some time later.
Parodied in Birds of a Feather, where Dorien had a habit of just walking into her neighbors' house. At one point, they locked the door. She smashed the glass panel next to the lock, undid it, and came in anyway.
Buffy the Vampire Slayer. Becomes a Running Gag in Season 4 when different characters keep wandering into Giles' home, with him lampshading by saying "I thought I locked that door." or "Now, I know I locked the door that time!"
When Angel becomes CEO of the supernatural law firm Wolfram & Hart, Eve (his Liaison to the Senior Partners) keeps doing this to upstage him, except for one episode when she does this while Angel is taking a shower. At first Eve claims his door was open, then changes to "your door was unlocked". Then Eve admits she has a key. Which Angel wordlessly takes off her.
This was a Running Gag on Even Stevens with the character Beans. Lampshaded in The Movie when the family is sitting down to dinner and Beans pops up, prompting Donny to say "This kid is starting to freak me out" and another character to propose changing the locks. Possibly averted by the fact that everyone found Beans weird and unsettling.
Smallville - this happens so often that characters Lampshade it. Particularly nonsensical for Lex Luthor, who has cameras and security systems. This is a Justified Trope when Clark is involved, but not when it's, for example, Jimmy Olsen.
Jackson: Jack? O'Neill: He's a barber. Jackson: Broke into your house? O'Neill: Yeah... Jackson: Second week in a row. O'Neill: Mm-hmm. Jackson: Alarm. O'Neill: I'm thinking "dog". Joe Spencer: You could try locking your front door.
Steve Urkel on Family Matters treated the Winslow house as his own. When asked at one point why he never knocks, he said that if he did, no one would let him in.
NCIS, any character that wants to speak to Gibbs when he's at home can, due to the fact that his front door doesn't even have a lock.
Done frequently on The Secret Life of the American Teenager. A notable example happened in the season one episode, "Maybe Baby" when nearly every regular character ended up in Amy's house, arriving in groups of two or three, just to tell her they would support her if she wanted to keep her baby. They have gotten better about it in later seasons (meaning it's more common to see people actually knock on the door and wait for someone to answer it) but it's not completely absent.
MythQuest: Philthy walks in on people a lot, often coming surprisingly close to learning about The Masquerade. Cleo asks him if he ever knocks and his response is, "Hmm, no."
A The New Yorker cartoon from 2002 has two agents walk into someone's house saying "We're from Homeland Security. You left your door unlocked."
In the town of Churchill (Manitoba, Canada), it is considered common courtesy to leave your house door and car door unlocked, just in case a stranger needs a place to hide from a rampaging polar bear. Car theft is not considered a problem as the town has no roads linking to the rest of Canada.
Though most RPGs use this, Golden Sun gets a special note for actually having bandits lampshade this and take advantage of a town's lack of home security to loot from the mayor, several homes, one of the protagonists and a holy sanctum.
In the opening cutscene of Super Paper Mario, Mario and Luigi simply walk into Bowser's castle.
Bowser: What? My front gate was open?! How many times have I told those idiots?! If you're the last in, LOCK THE GATE!
Homestar Runner: The title character finds his way into Strong Bad's house on many, many occasions. Even though Strong Bad is outspoken in his hatred of him. At one point Homestar lampshaded it by questioning whether he actually lived in Strong Bad's house.
Strong Bad: Is there like a sign on my door that says, "Wanted: Everyone I hate. Inquire within"?
King of Town: [randomly walking in] I didn't see one.
In an episode of Aqua Teen Hunger Force, when Dr. Weird's latest invention is stolen away through the hole caused by his previous invention.
Dr. Weird: (Standing in front of a gaping hole in the wall) "Who could have penetrated my impenetrable fortress?"
Done in Winnie-the-Pooh. The titular Pooh has set up an elaborate, if hasty furniture barricade in front of his Front door and is patrolling it with a popgun rifle. He hears a knock and is nervously trying to ascertain the identities of his friends knocking at the door, when Tigger bounces the bear, with Rabbit and Piglet walking in behind.
Pooh: How did you get past my Hunny-napper traps?
Rabbit: Your back door was open.
"Your back door was open" is used several times in the animated movies & episodes.
In an early episode of The Simpsons, when Homer gets an illegal cable hookup the cable guy comes by later with some stolen merchandise he thinks Homer might be interested in. "Your door wasn't locked in any serious way."
In one episode of Phineas and Ferb, Doofenshmirtz sets a trap for Perry the Platypus when he comes in, only for Perry to come in through the backdoor which the maid left unlocked.