For luck.
In Star Wars, the part where Luke and Princess Leia were going to rope swing across the huge void, Luke kept missing the landing, and they had to reset the scene each time. The kiss for luck was made up for frustration over spending the day hanging from a rope. They left it in.

Robert Downey Jr.‘s “The truth is…. I am Ironman.” line.

“Are you high”

If you’re just talking movies in general, in my all-time favorite dark comedy, “Doctor Strangelove”, all of George C. Scotts lines are ad libbed. Kubrick had asked Scott for a dry run and told him to just cut up and try to be funny, but that the actual script had his character, Air Force Chief of Staff General Buck Turgidson, written as a serious character. But the final cut left all of the so-called BS lines in the film. Scott was furious with Stanley Kubrick, but his lines are hilarious.

https://youtu.be/bZm3qBdECdw General Buck Turgidson (Dr. Strangelove)

Dustin Hoffman ad libbed this most famous line from Midnight Cowboy

The other night I watched You’ve Got Mail again.
There is a moment when Joe Fox (Tom Hanks) is walking out the door of a bookstore while he is carrying books, balloons, and a goldfish in a plastic bag, and the door traps a balloon and string.
He reopens the door and says “I’m glad that wasn’t the goldfish!”

Brad Pitt, as Tyler Durden, yelling “Run, Forrest, Run!” in Fight Club.

Marty Feldman playing Igor in Young Frankstein

The shifting hump on Igor’s back was an ad-libbed gag. Marty Feldman had been surreptitiously shifting the hump back and forth for several days when cast members finally noticed. The rest of the ad-lib of course his line “What hump ?”

In Ghost, Whoopi Goldberg ad libbed the line “Molly, you in danger girl.” Probably the most entertaining scene in the movie, and made even better by that line

Also, Joe Pesci improvised the conversation in Goodfellas. “How am I funny? Like a clown?”

Not a line, but a scene. Indiana Jones facing the big assassin in black with the giant sword. Indiana pulls out his gun and shoots him. Not what the original script called for, but Harrison Ford was sick that day.

At the end of Harry Potter and The Chamber Of Secrets, Lucius Malfoy improvised the line, “Well, let us hope that Mr. Potter will always be around to save the day.”
To which Harry improvised, “Don’t worry, I will be.”

Jason Isaacs later commented how impressed he was that a boy stayed with the story and made that response off the cuff without missing a beat.

There is a scene in Wild Hogs that I told Rick that I bet it was ad libbed. I just checked it and I was right.
“The scene in the Del Fuego’s bar, when Woody (John Travolta) was squinting, was almost entirely improvised. The cast had done the scene a couple of times and would ad-lib occasionally. One time John Travolta just started squinting in a Clint Eastwood impression. The other characters’ reactions were real. They were expecting John to say his lines, but (he) kept going with it.”