Some of the most famous scenes in many popular movies were the result of a spontaneous ad-lib.
The Godfather-1972
No one knows where the hell Brando got the cat come from. It was never in the script.

The Fugitive-1993
Tommy Lee Jones famous answer to Harrison Ford's plea in the sewers that he didn't kill his wife was simple but perfect, and entirely ad-libbed. "I don't care."

The Dark Knight Rises-2008
The slow clap of a creepy clown was never meant to be but only the dark brilliance of Heath Ledger could make it so.

Pretty Woman-1990
Snapping closed the lid of the jewelry box was a playful spontaneous move by Richard Gere but Julia Robert's reaction was so perfectly honest, they decided to keep the scene.
To see a much larger and mind blowing list hop on over to
Screen Rant.
And remember...life is meant to be ad-libbed.