"I am Iron Man" was ad-libbed by Robert Downey Jr., with Kevin Feige later crediting this moment as the catalyst to do away with secret identities in the Marvel Cinematic Universepic.

Source: https://comicbook.com/movies/news/iron-man-15th-anniversary-mcu-marvel-fans-celebrate/

It certainly feels ad-libbed with how much Dustin Hoffman's voice changes in the moment from his character, but with how great of an actor he is, fooling everyone would be easy.

Source: https://collider.com/midnight-cowboy-dustin-hoffman-cab-scene/

She ad-libbed jokes and sassily dedicated a Disney song to Florida Gov. Ron DeSantis.

Source: https://m.startribune.com/i-want-you-to-hear-me-hildie-edwards-has-become-minnesotas-foremost-trans-youth-advocate/600295416/

The show began with a scene between Hero, Beatrice and Leonata with the latter confirming that the war had ended and Claudio would be back but straight away she gave up with her script and ad-libbed her way through.

Source: https://www.thescarboroughnews.co.uk/news/people/sht-faced-shakespeare-hilarious-comedy-show-delights-crowd-and-proves-chaotic-at-scarborough-spa-4365153

They ad-libbed their entire scene, inventing the story about Tomas and his wife.

Source: https://screenrant.com/friends-robin-williams-cameo-explained/

When Christina Aguilera recorded her 2002 hit "Beautiful," the opening line "don't look at me" was completely ad-libbed.

Source: https://www.buzzfeed.com/kaylayandoli/music-facts-famous-women-remix