The five main lead actors also play their respective doppelgangers.
In "Bedtime Stories", Barney tells the story of how he once met all the "players" of New York City, all also played by Neil Patrick Harris.
Acting in the Dark: Cristin Milliotti didn't know that her character was going to die until they were filming the episode in question.
She didn't even know she was auditioning for the role of the Mother until after they cast her. Plus, she'd barely even seen the show before then.
The episode "Bad News" was scripted to end with Lily telling Marshall she was pregnant. The producers secretly told Alyson Hannigan right before shooting the final scene that the episode was actually going to end with her telling Marshal that his father had suddenly died of a heart attack. This means that Jason Segel's stunned and tearful reaction is genuine.
Like Robin, Cobie Smulders is from Canada. She was originally meant to be from Toronto, but changed to Vancouver to match Cobie's hometown. One episode also features Robin trying to get dual citizenship, something Cobie already has to allow her to work in both America and Canada.
A subverted example. Robin is eventually discovered to be infertile. Cobie Smulders was diagnosed with ovarian cancer in 2007 and it was ruled she wouldn't have children. She however later had two.
Most fans had decided that the Mother was Cindy's Closet Key, long before it was shown to be true.
Ted's kids were nicknamed "Luke and Leia" by some fans, based off Ted being told to swear on his future children and responding "I swear on Luke and Leia." Season 9's "Unpause" reveals that Ted's son is indeed named Luke, although the daughter is named Penny.
Mild example, but Jason Segel had said in interviews that he was ready to leave the show after the eight season, both because he wanted to pursue other acting roles, and because he felt there wasn't much left to do with Marshall as a character. He then suggested it would be interesting if Marshall and Lily got divorced, but then acknowledged that such a dark plot wouldn't work within the standard sitcom conventions the show utilized.
Alyson Hannigan wasn't happy at all with how the show ended, saying it was like slapping the fans in the face.
Creator Killer: The controversial series finale damaged Craig Thomas' and Carter Bays' reputation which many viewers vowed to never watch any of their works ever again. It could explain why many people lost interest on the planned spin-off How I Met Your Dad which never got picked by CBS and other networks. It doesn’t help that when the script of the pilot was leaked online, it’s more of a carbon copy of the HIMYM’s pilot episode.
The gang frequently plays their younger selves in flashback sequences, even to when they're teenagers.
In a meta-example James Van Der Beek played his character as a teenager. Possibly the only example where Dawson himself participated in Dawson Casting outside of Dawson's Creek. James Van Der Beek guest starred as Robin's old boyfriend and played both his teenage self and his middle-aged self.
Inverted with Robin who is actually a couple of years older than Cobie Smulders is in real life (presumably to shift her being a news anchor out of Improbable Age territory). Lampshaded in one episode, when Barney claims Robin always says she's two years younger than what she really is.
Defictionalization: Full versions of the music videos for Let's Go to the Mall, Sandcastles in the Sand, and P.S. I Love You have been created and the songs are up for purchase on itunes. Let's Go To The Mall is also available on Just Dance 3. The Bro Code and The Playbook were also made into actual books, plus Barney's suit-jamas are available online.. Also, an in-universe example when Robin describes the building Ted designed as "a building in your head that became real."
Almost all the websites mentioned on the show have been created by the show's production team. A partial list can be found on Barney's blog, which also counts as one.
People associated with Robin are all played by non-Canadian actors/actresses.
Jessica Glitter, Robin's co-star from her Canadian children's television show (although it's never stated directly that she's a Canadian citizen), is played by Nicole Sherzinger, an American who was born in Hawaii and raised in Kentucky.
Katie and Genevieve, Robin's sister and mother, are respectively played by American Lucy Hale and British Tracy Ullman.
Both actors to play her father Robin Sr., the first being German-American Eric Braeden and the second being American Ray Wise.
A popular theory online was that Ted would marry Barney's half-sister. In "Ring Up," Barney unknowingly encourages Ted to sleep with her and Ted unknowingly does, leading to Barney freaking out at the revelation, and then trying to get them to marry to justify it. They don't get married, but they do have fun teasing Barney…
Goes a step further. The actresses who were contacted to audition for the role of The Mother were actually screentested under the guise of a casting call for an actress to play Barney's half-sister.
"How Your Mother Met Me" is the timeline of the series from the perspective of The Mother, filled with little winks to fan theories. One in particular was the St. Patrick's Day party from "No Tomorrow" being revisited with the Mother's friend Kelly saying she could bump into the love of her life, with a cut to the shot in the original episode of Ted bumping into a random girl, the "bump girl" was a popular theory that she was supposed to be the Mother.
Luke and Leia for Ted's kids, based off Ted being told to swear on his future children and responding "I swear on Luke and Leia." Season 9's "Unpause" reveals that Ted's son is indeed named Luke, but his daughter is named Penny.
In the light of the final episode, there are many nicknames for the series such as "How I Met Your Step Mother", "How I Asked Your Permission To Bang Your Aunt Robin", "How I Met My Children's Surrogate Mother" and so forth.
The Mother is often dubbed as The (or simply) Uterus due to the above.
The TV show ending and the alternate ending are respectively called "The Blue French Horn" and "The Yellow Umbrella" endings, referring to Robin and The Mother's respective Arc Symbols in which depends who ends up (implied in Robin's case) with Ted.
Gratuitous Foreign Language: In "The Best Man" comments on Shmosby videos are actually intelligible Finnish, but obviously created with a translation software.
I Knew It: Many twists have been accurately predicted over the course of the series:
Marshall would use the final slap to stop Barney from freaking out before he got married.
The mother's name being the same as the stripper from "Belly Full of Turkey" was correct.
The fact that the mother was Dead All Along was a common fan theory even before the show got heavy with Foreshadowing.
Perhaps most surprising of all, Ted winds up trying to get together with Robin in the end. Despite marrying them both off to other people, they still wind up together… ish.
David Burtka, who plays Scooter, is Neil Patrick Harris' partner in real life.
Alyson Hannigan's husband, Alexis Denisof, plays Sandy Rivers, and their Buffy the Vampire Slayer co-stars Seth Green & Tom Lenk also appeared. Danny Strong guested as the minister at Marvin Eriksen's funeral.
Jason Segel's Freaks and Geeks co-stars Samm Levine, Martin Starr & Busy Phillips appeared over the course of the series.
Cobie Smulder's husband, Tarran Killam, appears as Gary Blauman.
Irony as She Is Cast: Neil Patrick Harris didn't officially come out as gay until a few years into the show (it was sort of an open secret beforehand). This made Barney's aggressive heterosexuality and his issues with his gay brother James even funnier. There is a whole ordeal with Barney opposing James' marriage, not because it would be a gay marriage but he was opposed to marriage in general.
Also Cheryl, Barney's biological father's wife is portrayed by two different actresses in her appearances. Nancy Travis (For Legendaddy) and Nancy Lenehan (in Season 9). This was due to Travis being busy with Last Man Standing.
Robin's dad was played by Eric Braeden in the character's first appearance in season 4, then was recasted with Ray Wise. The story goes that Braeden was offered to reprise the role but had a schedule conflict with a surgery appointment (which was minor, and considered a cop-out by the production). Ray Wise played the role for the remainder of the series.
Despite Ted, Lily and Marshall being the same age in-universe, Jason Segel is six years younger than Alyson Hannigan and Josh Radnor.
Alyson is likewise only fourteen years younger than Chris Elliot, who plays her father.
A justified example. Cristin Milioti is only two years older than her on-screen daughter Lyndsy Fonseca. Justified because Cristin only appears as Tracy in her youth while she was alive.
The Producer Thinks of Everything: The final shot of the series was filmed in 2005, to ensure that the children would still be children whenever the show ended. What's more is that the first Thanksgiving episode shows that the mother's name was decided in advance. The children's shocked reactions when Ted leads them to believe the stripper is their mother happen because she is called Tracy.
Josh Radnor is very allergic to dogs, and didn't tell the show's creators this until after he'd been hired. They worked around it for awhile with, according to a blog post Radnor wrote, "some Claritin and some thorough vacuuming." Eventually Robin sent her dogs to live somewhere other than a cramped New York apartment as part of the story, never to be seen again (though you can still see an affinity for dogs in decorations around her house, or occasionally on T shirts she wears).
Lily and Marshall rarely kiss on screen because Alyson Hannigan disliked Jason Segel's smoking habit, and was reluctant to kiss him too often.
The gag of Barney taking a magic trick through airport security and ending up in airport prison is something that happened to Neil Patrick Harris in real life.
Tuckerization: The bar McLaren's and the bartender Carl are named after the associate producer Carl McLaren.
Unintentional Period Piece: Given that the series is entirely told via Flash Backs, it's noted that time passes particularly with some of the pop culture references thrown during conversations and the evolution of the characters' cellphones (from the flip phones to PD As to smartphones). Lampshaded at the beginning of "Mystery VS. History", where a flashback in 2005 shows the gang debating about the most popular food in America and then in 2011, the gang are browsing through their smartphones where Robin searched that the most popular food is bread.
Also lampshaded in the parts of "Trilogy Time" that took place during Seasons 2 and 5.
It is never stated in the show, but the creators have confirmed that Carl's last name is MacLaren and that he owns the bar. This is a Shout-Out in and of itself—Carl MacLaren is a member of the HIMYM crew.
If the show wasn't picked up beyond the initial thirteen episode order, Victoria would have been The Mother.
Word of St. Paul: Prior to the series finale airing, Cristin Milioti was asked about the fan theories that the Mother has been dead the entire time Ted has been telling the story, and her response of "That's insane." was taken as a debunking of the theory. It did, however, turn out to be entirely accurate.
Jennifer Love Hewitt was initially slated to play Robin but backed out because she ultimately chose to appear in Ghost Whisperer. Likewise, Scott Foley was initially slated to play Ted. Jim Parsons also auditioned to play Barney right before scoring his breakout role on The Big Bang Theory. An episode of The Nerdist revealed that Gillian Jacobs auditioned for Robin. Chris Hardwick also went up for a part. Samm Levine also claims he was auditioned repeatedly for Barney, but felt he was wrong for the part.
Alicia Silverstone was originally cast as Stella but backed out at the last minute. Rumour has it that when she heard Britney Spears would also be guest starring, she didn't want her appearance to be overshadowed. Sarah Chalke replaced her.
If the show hadn't gotten picked up for a second season, Victoria would have been the mother. Had it not been renewed for a fourth, then Stella would have been.
Season 8 was intended to be the final season, but when the show was unexpectedly renewed for a ninth season the planned ending (Robin and Barney get married and Ted meets the mother) had to be pushed back.
Within two weeks of the episode airing, Carter Bays and Craig Thomas still hadn't decided on the ending of the series, trying to choose between two endings. The alternate ending was on the season DVD release.