The scene where Mrs. Doubtfire covered her face with icing didn't go to plan when the set lights began melting it off Robin Williams' face. When the icing fell into Mrs. Sellner’s tea, Williams decided to run with it and improvised the line “There you go, you’ve got your cream and your sugar now.”

The moment in Guardians of the Galaxy when Peter Quill drops the orb was unscripted and just the result of actor Chris Pratt's clumsiness. But since Pratt still stayed in character and the scene looked good, it stayed in the film.

The line in Forrest Gump where he introduces himself with "My name’s Forrest Gump. People call me Forrest Gump" wasn't in the script. Tom Hanks said the last part by accident and they just left it in.

The iconic scene where Ross says "Rachel" instead of Emily, during his wedding to the latter was actually not in the original script.

David Schwimmer had mixed up the names in another scene and the writers decided to use it for this gasp-worthy moment in Friends.

In Princess Diaries, Anne Hathaway plays Mia, an awkward and geeky girl who discovers that she's the heir to the throne of Genovia. Hathaway actually fell in one of the scenes where Mia was supposed to walk on bleachers. This blooper fit the clumsy character really well so they kept it in.

The Dirty Dancing scene where Johnny and Baby crawl toward each other on the floor was just part of the actor's warm up before filming. But the directors loved it and decided to make it part of the film.

The scene in Season Three of Stranger Things where Eleven collapsed into Michael’s arms and bursts into tears after using all her energy to fight, was just a genuine reaction from actress Millie Bobby Brown, after intense days of shooting. 

However, since Eleven collapsing at that instance seemed very natural, the scene was kept in.

Anthony Hopkins stole the show as Dr. Hannibal Lecter in just 25 minutes of screen time in The Silence Of The Lambs. Most memorable is the scene when Dr. Lecter is talking to an FBI agent and he starts to hiss — this was fully improvised and resulted in completely genuine reactions from his costar.

The scene in Thor: The Dark World where Thor hangs his hammer up on a coat rack was simply Chris Hemsworth goofing off. But his improv turned out so good, that the scene was left in the final cut of the film.

Sometimes keeping actors unaware of an incoming plot twist allows them to have genuine reactions that pay off well. One such scene is in How I Met Your Mother where Marshall finds out his dad died of a heart attack. Actor Jason Segel didn't know that the character’s dad was going to pass away until they started filming the episode, so Marshall's reaction had a lot to do with Segel's own shock.

In this fan-favorite moment in The Office, Michael was not meant to kiss Oscar. But Steve Carell took things into his own hands and went for a kiss on the mouth of his costar for a masterpiece improv moment.

Neil Flynn, who played the unnamed Janitor on Scrubs, was truly a masterclass on improvisation. It's Flynn's incredible wit that made Janitor — who was only meant to appear in the pilot — a permanent fixture in the series. 

Almost everything that Janitor said across eight seasons of the show was all Flynn and had nothing to do with a script.

Eternal Sunshine of the Spotless Mind is quite a trippy film. One scene in which Kate Winslet’s character disappears abruptly is given a genuine element of surprise because Jim Carrey wasn’t informed of it — so his expression is natural.

Sherlock is nothing if not sharp and clear with his words. But in a scene where he and Watson get drunk, Benedict Cumberbatch's fumble with his lines fit perfectly though it wasn't actually in the script.

Dr. Gregory House, the titular character of the medical drama, House, was well-known for his snarky wit. However, many of the grumpy doctor's one-liners actually came from the actor Hugh Laurie, who began improvising them quite often as part of his comedic background.

In iconic slasher film, Scream, the scene where Matthew Lillard’s character finds his gun gone sees him react with the line, “Ah... Houston, we have a problem.” As scripted as it seems, this was actually an ad-lib by the actor.

Heath Ledger in his last role as Joker is a whole different level of brilliance. The actor added several mannerisms and improvised actions to make the character his own. One iconic scene is where Joker claps sarcastically during Jim Gordon’s promotion scene — this was done by Heath completely in the spur of the moment.

The famous scene in Godfather, where Marlon Brando’s mafia boss character,   Don Vito Corleone, gives out brutal orders while softly petting a cat was just a perfect accident. The cat randomly showed up on set and jumped into Brando's lap as he was doing the scene, making it so much more memorable than anyone could have imagined