That funny line from Ant-Man and the Wasp: Quantumania where a worker confuses Ant-Man for Spider-Man was actually an accident — the actor confused the two characters.
Speaking of Marvel...in Guardians of the Galaxy, Peter wasn't meant to drop the orb in the scene where it slips from his hand and he quickly catches it. That was actually Chris Pratt accidentally dropping and catching it.
In A Knight's Tale, the crowd was supposed to cheer at Chaucer's speech. However, many of the extras did not speak English and didn't realize it was time for them to cheer. Mark Addy, who played Roland, then started cheering to get the extras to join in — a funny moment that director Brian Helgeland kept in.
The assistant cameraperson accidentally opened the camera magazine in one of the last scenes of The Last Temptation of Christ, causing there to be exposure from light on the edge of the film. However, director Martin Scorsese ended up loving it, saying, "The edge fog became the Resurrection."
In Scrooged, Bill Murray's character, Frank Cross, wasn't supposed to fall after throwing water on the waiter. Murray accidentally slipped on the water on the floor, and it was so funny that they kept it in.
Maria's trip in "I Have Confidence" in The Sound of Music was also unscripted, but it ended up being the perfect amount of irony for the number, while also demonstrating how poorly prepared the nanny, played by Julie Andrews, actually was.
In Almost Famous, William asking Penny to ask him to come to Morocco with her again was actually Patrick Fugit asking Kate Hudson to give him the line again because he wanted to say his line again, only more excited. However, this take — where he becomes more excited after the second time she asks the question — was so sweet that it made it into the final cut.
According to the commentary for American Beauty, the remote control car hitting Carolyn, played by Annette Bening, was a complete accident — but it was so perfect, it ended up in the film.
The sound of lightsabers in Star Wars was actually created through a happy accident. When trying to come up with how they'd sound, sound designer Ben Burtt walked by his TV with a tape recorder that had a broken mic cable, and it picked up the sound and amplified it, creating a buzzing noise that Burtt used in the film.
Terry, played by Charles Martin Smith, wasn't meant to crash his scooter in American Graffiti — that was a funny accident that made it into the film.
The crash heard after Uncle Billy walks offscreen in It's a Wonderful Life wasn't planned — a bunch of props were dropped right as he walked offscreen, and Thomas Mitchell (who played Billy) improvised "I'm alright!," pretending the crash was him. James Stewart's laugh is real.
The "tears" reflected on Robert Blake's face in In Cold Blood were actually by accident. Conrad L. Hall, the director of photography for the film, noticed the rain effect as he was setting up the lighting of the scene using a stand-in, and he decided that it was perfect for the scene.
Alvy's sneeze — sending his friend's expensive cocaine into a cloud — in Annie Hall was unscripted and accidental. But when Woody Allen, who played Alvy, sneezed, it was decided that ruining his friend's coke was so typical of Alvy, and funny, that it stayed in the film.
In the scene from The Mask when Stanley, played by Jim Carrey, tries to stuff the money in the closet, Milo wasn't supposed to bite the Frisbee. But the Jack Russell terrier actually did jump and bite the Frisbee, holding on for dear life. This was so funny that they decided to incorporate it into the movie.
Another funny scene that wasn't meant to happen was when Joel threw the arrow on the ground in Addams Family Values. The actor, David Krumholtz, couldn't get the arrow on the bow. "So in real-life frustration, that's me throwing it down and walking out," Krumholtz said. "I was super angry at myself. Then, after they cut, they all started laughing, and they were like, 'That was hilarious!' I was like, 'Oh! OK!'"
The cigarette ash wasn't supposed to hit McManus in the eye in The Usual Suspects — it was an accident, and Stephen Baldwin's reaction is real.
And the laughing in the lineup scene wasn't supposed to happen either — it was meant to be serious. “We were supposed to be very stoic, very unimpressed, unintimidated,” Kevin Pollak, who played Hockney, said. But “we would lose it over and over and over.” Pollak also claimed in the DVD commentary that part of the reason they laughed was that Benicio del Toro, who played Fenster, "farted like 12 takes in a row," which created one of the most memorable scenes in the film.
Similarly, Dustin Hoffman actually accidentally farted in the phone booth scene in Rain Man, and Tom Cruise's reactions were improvised. Hoffman called it his "favorite moment" of any film he's done: "That includes Shakespeare that I've done onstage, anything."
The tender moment when Adrian, played by Talia Shire, hesitates to kiss Rocky actually occurred because Shire was sick. She was anxious about kissing Sylvester Stallone because she didn't want to get him sick, but her reluctance ended up being perfect for the scene.
Similarly, Toothless hesitating to touch his nose to Hiccup's hand in How to Train Your Dragon was a mistake — it was caused by a software error — but it fit the moment, so it was kept in.
The Children of Men scene when blood splatters on the camera was a complete accident — it happened on their last chance to film the six-minute continuous scene. Director Alfonso Cuarón yelled "cut" when the blood splattered onto the camera, but the assistant director didn't hear him because an explosion went off at the same time. They continued with the shot, which the cinematographer called a "miracle."
The stuntperson filming the scene of McClane, played by Bruce Willis, falling down the elevator shaft in Die Hard actually lost his grip, leading to the fall being much longer and scarier than it was supposed to be.
Timothy Dalton, who played Simon Skinner in Hot Fuzz, accidentally looked at the camera in this scene. According to the DVD commentary, the moment was kept in and accompanied with a "ka-ching" to make it seem intentional and meta.
Actor Patrick Scott Lewis, who played Bryan Hartnell in Zodiac, actually had indigestion during one scene. The scene where he holds his stomach was kept in because director David Fincher thought it made his fear seem more realistic. "I just thought that seemed so real, that somebody would actually kinda [have] a moment of indigestion when they see a guy in a black hood and a .45 automatic," Fincher said in the commentary.
The helicopter crash in Attack of the Killer Tomatoes was not meant to happen. It really did accidentally crash and burst into flames...which was caught on film. So they decided they might as well use it.
The cat that Don Corleone, played by Marlon Brando, creepily (yet iconically) pets in The Godfather was not in the script. Director Francis Ford Coppola spotted the stray on set and handed it to Brando (who loved animals) for the scene. In fact, it almost ruined multiple takes from purring too loudly.
And finally, Lenny Montana, who played Luca Brasi, was so terrified to act alongside Brando that he tripped up on his lines in the scene where his character greets Don Corleone. Coppola saw this and decided to make his anxiety a part of his character, adding in an additional scene in which Brasi practices greeting Don Corleone. This made Brasi far more layered and memorable.