🚨Warning: Some mild spoilers ahead! 🚨
1. In Parks and Rec, Chris Pratt improvised Andy's line about typing Leslie's flu symptoms into the computer.
2. In The Office, Steve Carell surprised Oscar Nunez by kissing him on the lips during the episode "Gay Witch Hunt." In the script, Michael was supposed to kiss Oscar on the cheek.
3. In Riverdale, when Archie was trying to save Cheryl from drowning in Sweetwater River, K.J. Apa actually broke his hand while pretending to punch through the ice.
4. In The Fresh Prince of Bel-Air, Will's speech about his dad not wanting him was completely improvised by Will Smith.
5. In How I Met Your Mother, Jason Segel didn't know Marshall's dad was going to die until they shot the episode, so the final cut consists of his genuine reaction.
6. In Buffy the Vampire Slayer, Seth Green and Alyson Hannigan improvised the memorable scene in which Oz and Willow joke about animal crackers.
7. In Supernatural, Jensen Ackles once screamed Dean's classic "son of a bitch" line and made Jared Padalecki laugh on camera. The line wasn't in the original script.
8. In Parenthood, Craig T. Nelson and Mae Whitman improvised the entire scene in which Zeek takes Amber to see her wrecked car after her horrific car crash.
9. In Parks and Rec, Patton Oswalt's filibuster monologue was completely improvised. He was given free reign, so he talked about a proposed Star Wars and Marvel crossover film.
10. In The Walking Dead, Danai Gurira improvised an entire scene where Michonne killed a bunch of walkers. The director didn't yell cut, so she kept on going until they were all dead.
11. In Orphan Black, the entire scene in which Alison and Donnie buried a body in their garage was unscripted. The director set up a camera and let Tatiana Maslany and Kristian Bruun do whatever they wanted.
12. In Agents of S.H.I.E.L.D., Iain De Caestecker added in a reference to Breaking Bad during the Season 2 finale when he uttered Jesse's iconic catchphrase.
13. In Friends, when Ross played the bagpipes, part of the blooper made it into the final cut. If you look closely, you can see Jennifer Aniston laughing at Lisa Kudrow singing.
14. In some episodes of Scrubs, Neil Flynn improvised his own lines as Janitor. The script would simply read, "Whatever Neil says."
15. In Timeless, Malcolm Barrett worked the term "clockblockers" — what fans of the series call themselves — into the show, even making it a curse word.
16. In Sherlock, Benedict Cumberbatch forgot one of his lines while filming a scene in which Watson and Sherlock get drunk, but it worked, and Cumberbatch's improvised line made it into the final cut.
17. And finally, Stefon's iconic pose, with his hands covering his mouth, was Bill Hader’s reaction to reading new lines John Mulaney came up with moments before a taping. Hader couldn't contain his laughter on camera.
Will's speech on The Fresh Prince of Bel-Air was not inspired by Will Smith's real-life father. An earlier version of this post said that it was.