1. According to a story Jane Russell recounted in Blonde Heat: The Sizzling Screen Career of Marilyn Monroe, Russell was not supposed to fall in the pool during “Ain’t There Anyone Here for Love?” in Gentlemen Prefer Blondes. One of the dancers, Ed Fury, accidentally pushed her in with his foot. He was then fired because he tried to claim co-choreographer credit for the mistake.
In ‘Gentlemen Prefer Blondes’ (1953) Jane Russell’s pool sequence was supposed to end with a muscleman diving over her, but she was accidentally clipped by his foot and knocked into the water. “I wasn’t supposed to end up in the pool at all,” she later said, “but it turned out better that way.” from MovieDetails
2. Costar Hank Azaria revealed that Robin Williams' fall in The Birdcage was accidental. "If you watch that little piece of film again, you'll see me laughing and Robin laughing. It's one of those things that happens that you never really think they're going to use, but I was so emotionally upset in the scene — I was supposed to be crying — that I just pretended that he was making me cry even more. But I was actually laughing."
3. Joaquin Phoenix studied wild animals in captivity for his prison cell scene in The Master. Smashing the toilet wasn't in the script; it was just a part of him going berserk. "I didn't intend to break the thing," Phoenix said. "I didn't know that was possible."
4. In The Way of the Gun, Ryan Phillippe actually did punch Sarah Silverman by accident. He didn't knock her out as the caption says, but he did knock her off her feet. Philippe felt so bad that he immediately started crying.
5. Similarly, Harrison Ford actually punched Ryan Gosling in Blade Runner 2049 due to the strobe lights making it hard to see. Ford's reaction is real — he described it as “That’s the face of, ‘Oh sh–, I’m in deep doo-doo.'”
6. And Thomas Jane actually stabbed Kevin Nash in The Punisher, because a stunt coordinator forgot to change out his real knife for a prop knife.
7. The famous shot of Daniel Craig as James Bond walking out of the water in Casino Royale was not meant to happen. "It was actually by accident," he said. "Where we filmed, off the Bahamas, it's just one of those places where there is a sand shelf and the sand shelf happens to be three feet deep. Because the idea was, I was supposed to swim in and sort of float off, but I swim in and stand up. And it was just one of those things."
8. Michael McDonald insisted on Melissa McCarthy throwing a real phone book in The Heat, and said he would dodge it if it came near his face. But McCarthy ended up being positioned closer than he'd thought, and he didn't have time to dodge, meaning he was hit directly in the face.
9. Eva Green's hair accidentally caught fire in a kitchen scene in The Dreamers. Her costar Michael Pitt quickly extinguished it, and they kept going with the scene. The director kept it in.
10. In 1917, Schofield wasn't supposed to fall — an extra ran into him, causing him to fall over. He got back up again and kept running. Screenwriter Krysty Wilson-Cairns said she screamed when he fell, and that it was like "watching your team mess up a field goal at the Super Bowl." When George MacKay got up, director Sam Mendes yelled to keep filming, "and out of that came a sort of movie magic, I think, personally," Wilson-Cairns said. "I’m obviously a little biased, but it’s one of my favorite scenes of the film."
11. The cigarette ash wasn't supposed to hit McManus in the eye in The Usual Suspects — it was an accident, and Baldwin's reaction is real.
12. In Cheech & Chong's Up in Smoke, the dog that eats Chong's burrito was a real stray that just wandered up and ate the burrito. It wasn't in the script.
A movie detail for 4/20: in Cheech & Chong’s Up in Smoke (1978), the dog who eats Chong’s burrito when they are in Tijuana was unscripted. It was a local stray that wandered into frame, took the burrito, and walked away. Cheech & Chong just ad-libbed around it, and the scene stayed in the movie. from MovieDetails
13. Joel throwing the arrow on the ground in Addams Family Values was unscripted. 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!'"
In Addams Family Values (1993), the part where Joel throws an arrow at the ground wasn’t a part of the script. According to David Krumholtz, who played Joel Glicker, he actually screwed up during this scene, because he couldn't get the arrow notched, so he threw it at the ground out of frustration. from MovieDetails
14. Screenwriter Robert Towne revealed that John Huston genuinely mispronounced Gittes' name while filming Chinatown, and that the director, Roman Polanski, decided to keep it in. "That was Roman as much as it was Huston," Towne said.
In Chinatown (1974), Noah Cross (played by John Huston) repeatedly mispronounces Gittes’ name as “Gits” instead of “Git-iss.” This was not scripted, but was due to Huston’s inability to get the name right. Director Roman Polanski decided to keep it in, as it fit the nature of the character. from MovieDetails
15. The crashing 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.
It's A Wonderful Life (1946): In the scene when Uncle Billy drunkenly walks off, a loud crash is heard a moment later and Uncle Billy reassure George that he is alright. This scene was unscripted and happened due to a stagehand dropping a tray of props during filming. WARNING: Clip is loud. from MovieDetails
You can watch the clip here.
16. The "tears" reflected on Robert Blake's face in In Cold Blood was actually by accident — it was noticed by Conrad L. Hall, the director of photography for the film, as he was setting up the lighting of the scene using a stand-in, and he decided it was perfect for the scene.
17. 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 with a broken mic cable, and it picked up the sound and amplified it, creating a buzzing noise Burtt used in the film.
In the First Star Wars movie our Immediately Recognizable Lightsaber Sound was a Mistake...While carrying a tape recorder with a Broken Mic Cable the Shielding had Come Off and the sound was Recorded as he Walked by a Television...the Sound is the Electrical Feedback from the Television's Tube. from MovieDetails
18. The stuntman filming the scene of McClane falling down the elevator shaft in Die Hard actually lost his grip, leading to the fall being much longer and more scary than it was supposed to be.
In Die Hard (1988) the scene where John McClane falls down the elevator shaft was actually a mistake. The stuntman was supposed to grab the first vent, but slipped and continued to fall down the shaft. The footage was kept and edited together with McClane grabbing the next vent down. from MovieDetails
19. Kurt Russell accidentally smashed a real 145-year-old guitar instead of a replica in The Hateful Eight. The take ended up in the film.
20. According to Dangerous Days: Making Blade Runner, Daryl Hannah's slip in Blade Runner was an accident. She finished the take, then had her elbow looked at and found out it was chipped in eight places.
21. The fogginess at the end of The Last Temptation of Christ was accidental, caused by a mistake from the assistant cameraman, but director Martin Scorsese liked it so much he didn't reshoot it.
When the final scene of The Last Temptation of Christ (1988) was filmed, the assistant cameraman thought that he destroyed the best take by accidentally opening the camera. In editing, Martin Scorsese decided that the resulting edge-fog on the “destroyed” take should represent the resurrection. from MovieDetails