For a series that ended its run over 15 years ago, Friends almost feels like a show that is still on the air the way it remains such a strong presence in the pop culture zeitgeist. It's so popular that Netflix once paid a whopping $100 million to keep the show on the service for just one additional year.

One of the things that made Friends such a great show is in the talent and chemistry of its main cast, as well as its many memorable guest stars. And while its actual writing was some of the sharpest and snappiest in sitcom history, many memorable moments from Friends came in the form of improvised, unscripted, and otherwise unplanned moments that were left in and became a part of television history. While we all nurse our wounds from the delay of the reunion special, let's look back at some of those moments and how they came about.

15 The Show's Final Line Was Ad-Libbed

A shot from the Friends series finale
via friends.fandom.com

Other than the two apartments, the most commonly-visited location on Friends was the fictional Central Perk café that was seemingly the only place the gang ever hung out. So, in the series finale, when Rachel suggests the group go and grab a cup of coffee, Matthew Perry couldn't help but to ad-lib the sarcastic response of "Sure, where?"

14 "I Take Thee Rachel..."

Ross and Emily's wedding from Friends
via thesun.co.uk

This one is a bit of a cheat, as the shocking moment when Ross mistakenly says "Rachel" instead of "Emily" during his vows to the latter was very much in the script. However, writer Greg Malins stated in an interview that that big twist was inspired by a previous example of David Schwimmer saying "Rachel" when he was supposed to say "Emily," which was too good not to put to use later.

13 If Billy Crystal And Robin Williams Want To Riff, Let Them Riff

Billy Crystal and Robin Williams on Friends
via buzzfeed.com

Arguably the most memorable moment in Friends guest star history was the scene with Billy Crystal and Robin Williams. Turns out that the entire thing was completely impromptu, as the pair had simply stopped by to visit the set and were invited to do a cameo and agreed-- which involved the two improv legends just doing whatever. Not surprisingly, the off-the-cuff scene was pure gold.

Related: Zelda Williams Uncovers Old Photos With Late Father Robin Williams

12 Joey Didn't Hurt His Arm Jumping On The Bed

Matt LeBlanc as Joey and Matthew Perry as Chandler on Friends
via someecards.com

In "The One Where No One's Ready," much of that memorable episode is built around Chandler and Joey fighting over a chair. During one of their tussles, Matt LeBlanc actually got seriously hurt, despite an injury not being in the script as he revealed on Today. Later, when he had to film episodes in an arm sling, it was written that Joey hurt himself jumping on the bed.

11 "Oh My God, David!"

Ross, Ben, and Rachel of Friends
via thesun.co.uk

According to the book "Friends Forever: The One About The Episodes," Jennifer Aniston wasn't told in advance that Rachel was going to be pranked by Ross throwing a dummy of himself down the stairs. Producers wanted a more genuine reaction from Aniston, which is exactly they got-- including her screaming, "Oh my god, David!" which of course had to be dubbed over after the fact.

10 Paolo's Italian Lines Were Improvised

Rachel and Paolo from Friends
via metro.co.uk

When it came time for Rachel to have an Italian-speaking boyfriend named Paolo, the writers had two choices: To try and come up with actual lines in Italian even though none of them spoke it, or to just let actor Cosimo Fusco improvise his Italian dialogue. And, as Fusco revealed on Today, they went with the latter, meaning he ad-libbed everything he said in Italian.

9 Donald Trump's Blue Blazer...Black?

Matthew Perry flubs a line in Friends
via twitter.com/scenesinfriends

While hilarious, the scene where Matthew Perry delivers the line, "Donald Trump wants his blue blazer black" then has to correct himself and say "back!" seems a little awkward. And that's because it was an actual mistake that Perry made, and the way everyone made fun of him for the error and the way he responded to it was too funny to leave out, according to "Friends Forever."

8 Marcel Monkeying Around

Marcel the monkey and Ross in Friends

Using live animals in a TV show is dicey, and Marcel the monkey caused plenty of issues for the Friends cast and crew. Katie, who played Marcel, was mostly professional but still had her mischievous moments, some of which were too funny to leave out. According to her trainer in an interview with The Telegraph, one such moment was when Katie threw a bra at Jennifer Aniston.

Related: 10 Times Animals Were Total Jerks

7 Carol Cries For Real

Ross and Carol share a tender moment in Friends
via youtube.com (Favorite Videos)

The early seasons of Friends saw Ross struggling with the dissolution of his marriage to Carol. In the scene where Ross makes one last heartfelt plea for Carol to come back to him, actor Jane Sibbett said in an interview with Entertainment Weekly she wasn't instructed to cry but couldn't help but to do so at the tenderness of David Schwimmer's performance, which is the take they used.

6 Richard Branson Was Too Busy To Rehearse

Matthew Perry and Matt LeBlanc in a scene from Friends with guest star Richard Branson
via imdb.com

Friends could get anyone to cameo, including billionaire businessmen like Richard Branson. And while the scene in which Branson plays a souvenir cart vendor was fully written out, the ever-busy entrepreneur didn't have time to actually learn his lines-- so when he showed up to do his scene, he, Matthew Perry, and Matt LeBlanc just kind of made something up, as LeBlanc later told Entertainment Tonight.

Related: Richard Branson Is Offering $3 Million To Whoever Can Reinvent Air Conditioning

5 Chandler's Filing Cabinet Mishap Wasn't Planned

Matthew Perry as Chandler handcuffed in Friends
via imdb.com

In one of the most shockingly hilarious scenes in Friends history, Rachel walks into her boss, Joanna's, office to find Chandler handcuffed to a filing cabinet with his shirt unbuttoned. One of the funniest moments in that scene is when the drawer he is attached to opens and hits him in the head, which was a complete accident according to the DVD commentary.

4 "Not So Much Dating As Walking Bottles Of Vodka"

Matthew Perry as Chandler Bing on Friends
via favim.com

A behind-the-scenes special about Friends that aired on CNN revealed many interesting tidbits, including a moment where Matthew Perry workshops a better Chandler line. When "I don't think they're dating as much as they are drunk" fell flat with the studio audience, he came up with the funnier "I don't think they are dating as much as they are two bottles of vodka walking around together" on the spot.

3 "The World's Worst Hangover"

Jennifer Aniston as Rachel Green in Friends
via diply.com

Speaking of funny, improvised lines concerning Ross and Rachel's drunken Vegas wedding, Jennifer Aniston deserves credit for also coming up with an iconic, off-the-cuff zinger as revealed by the DVD commentary. During Rachel's rage as she comes to grips with what she's done, Aniston tossed off the ad-libbed line, "This isn't a marriage, this is the world's worst hangover!" which was one of the funniest things Rachel ever said.

2 Christina Applegate Struggles To Say "Phoebe"

Christina Applegate as Rachel's sister in Friends.
via fandom.com

Christina Applegate as Rachel's sister is one of the most perfect bits of casting in television history, and she was a standout among the show's many great guest stars. In fact, when Applegate's character hilariously struggles to pronounce Phoebe's name, the actress later revealed in an interview with Today that she really did have trouble with the name initially and it was too funny not to leave in.

1 "Stop The Q-Tip When There's Resistance"

Chandler delivers the famous "Q-tip" line to Joey
via every-friends-joke.blogspot.com

It's frequently mentioned among the best Friends lines, Chandler telling Joey: "Okay, you have to stop the Q-tip when there's resistance." And to give credit where credit is due to an absolutely perfect reply to another character saying something stupid, Matthew Perry completely ad-libbed that line on the spot, with the script only instructing him to scoff and walk away, according to "Friends Forever."

Next: If You’re Rewatching Friends, Watch Out For These Mistakes