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 Marshall that his father suddenly died of a heart attack. This was done to keep Alyson's and Jason Segel's stunned and tearful reactions as genuine as possible.
The actress who plays the Mother didn't even know she was auditioning for that role until after they cast her. Plus, she'd barely even seen the show before then.
The finale casually revealed the death of a recurring character. This revelation was as shocking to the audience as it was to the actor, who was not informed of the character's death until just before shooting. Significantly, the character in question was the titular Mother.
Actor Allusion: At the end of season 3 episode "The Bracket", Barney is shown writing on his blog, which looks remarkably like Doogie Howser's journal, and the theme tune even plays over the scene.
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.
While her nationality isn't explicitly pinpointed, Robin's mother is played by British Tracey Ullman with her accent intact. Cobie Smulders's mother is also British.
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.
In something of an inversion, Alyson Hannigan hates her singing voice and asked not to be given much to do there (a similar thing happened with the Musical Episode of Buffy the Vampire Slayer). With that in mind, you'll notice she is never given a complete sentence to sing, such as in "Girls vs. Suits," or given something else to do while another is singing like playing the xylophone, tuba and violin in Marvin's lullaby.
Cristin Milioti's ukulele skills and her beautiful singing voice are showcased in the 200th episode with her captivating rendition of "La Vie En Rose".
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.
Another mild example with Josh Radnor, who towards the end of the show expressed annoyance towards some of Ted's negative traits, such as his immaturity with relationships, propensity to Aesop Amnesia, and that his self-professed "Nice Guy" status at times was an Informed Attribute.
Alyson Hannigan wasn't happy at all with how the show ended, saying it was like slapping the fans in the face. She also said she was never happy with the Barney and Robin pairing, feeling it complicated the Ted and Robin relationship too much.
Creator Killer: The controversial series finale damaged Craig Thomas' and Carter Bays' reputation, to the point 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 was found to be more of a carbon copy of HIMYM's pilot episode. This is even more noticeable in their filmographies post-HIMYM, where they haven't been doing much asides from a bunch of forgotten TV movies.
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.
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 to play in Just Dance 3.
The Bro Code and The Playbook were also made into actual books, plus Barney's suit-jamas are available online.
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 Scherzinger, 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 Tracey 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 It 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 six reaffirmed those desires. 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 also got a new nickname: The Uterus.
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.
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 because was opposed to marriage in general.
Name's the Same: In "The Naked Truth," there's Lenny Kravitz, an architect so renowned Ted describes him as a rock star. That metaphor is notably deceptive to Robin and the audience because Robin thought the guest at The Architects' Ball was Lenny Kravitz.
One-Take Wonder: In the episode "Bad News", Lily steps out of a cab and tells Marshall that his father has died. This prompts an emotional reaction from Marshall, with him embracing Lily and crying "I'm not ready for this." Marshall's actor, Jason Segel, did not know about the twist before Lily said the the words, and the scene was done in only one take.
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 (2011).
Robin's dad was played by Eric Braeden in the character's first appearance in season 4, then was recast with Ray Wise. The story goes that Braeden was offered to reprise the role but claimed that he 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.
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 with her lesbian aunts on a farm since each dog was from previous boyfriends, and she had just made Ted get rid of things that belonged to his exes. The dogs were 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). However she had gotten new ones by the end of the series in the year 2030.
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.
Technology Marches On: In "Where Were We?", Lily is the victim of credit card theft by a coffee shop clerk. In the scene building up to it she is digging through her purse for cash while an annoyed line of people is forming behind her, before finally giving up and asking if they take credit cards. Using your credit card as a last resort can seem a little strange these days, as cash and credit cards are slowly switching places in terms of being the default payment method and some businesses completely excising cash altogether.
Tuckerization: The bar McLaren's and the bartender Carl are named after the associate producer Carl McLaren.
Given that the series is entirely told via flashbacks, 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 PDAs 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.
Some of the show's politics solidify it as a product of the mid-2000s. Starting with the mid 2010s, Ted's behavior towards Robin comes off less as love and more of a nasty case of Entitled to Have You. (i.e., Ted seems more like a self-proclaimed "Nice Guy" than an actually nice guy.)
Barney's Lovable Sex Maniac antics were already a bit iffy, and he wasn't exactly considered a great guy for them in-universe, but evolving views on consent, respect, and even rape progressively dated some of his conquests and made them come off as almost monstrous while in the show, no matter how terrible they were, they were played as a joke.
Ted admits to getting beaten up by one of his ex-girlfriends. His last two girlfriends, Zooey and Jeanette, would alternate between chipping away at his sense of identity and being outright violent for the sake of it. All these moments are Played for Laughs to some degree or another. The show was made around the tail end of the time where Double Standard: Abuse, Female on Male could be played for laughs, before it was more widely accepted as serious socially that men can be victims of domestic abuse as well as women.
The show has quite a few gay and transgender jokes that relay on stereotypes. The transphobic jokes in particular have not aged well, especially not the one that implies being a trans woman is on the same level as being a puppy-killer, and the one where Zoey outright uses the word "tranny," which is considered a slur when used by anyone outside the trans community. The show was made in about the last era where Queer People Are Funny-jokes could still be made, and it shows.
This is played with, however, for "Monday Night Football": despite the episode taking place in 2007, it isn't outright said. Also, because of Future Ted's poor memory about that particular Super Bowl, at no point are any team names mentioned.
Barney's suits also date the show almost precisely. At the time, suits were generally not worn by men, unless they had to, for work or formal functions. After the show ended, they made a comeback for normal wear, especially due to the rise of hipster culture. So, while Barney's obssessiveness is still quirky, his desire to "suit up" will not be seen as out of the ordinary by modern audiences, as was intended by the writer. The particular type of suits are another example; they are nearly all dark or grey, never light-coloured, popular both before and after the shows run, but not during. Barney also starts to wear three piece suits, popular in the late 2000's, early 2010's.
Alicia Silverstone was originally cast as Stella Zinman, but dropped out of the project. Reports at the time indicated that upon learning that Britney Spears would be playing her receptionist Abby, Silverstone's reps had her withdraw from the role because they did not want her first appearance on the series to be overshadowed by Spears.
Although the plot is the same, most of the dialogue and jokes in the first script for the pilot were rewritten. For example, some characters have slightly different quirks, like Marshall being portrayed as a bit of a hypochondriac who worries about having various diseases. The biggest divergence, however, is a missing scene when Barney, Lily and Marshall are waiting near Robins apartment: Marshall tries to prove that he is assertive and runs off to buy another champagne bottle. Once they start drinking they are caught by a police officer and Marshall is actually arrested when he tries to talk back. Lily has to bail him out and then they go to MacLaren's.
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.
Nick, whose return was promised by Future Ted, was originally supposed to appear as Robin's love interest in season 7. When his actor was unavailable, Kevin was created to take his place and Nick returned in season 8 instead.
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.
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.
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.
Writing by the Seat of Your Pants: The series starts with a Driving Question of "How does Ted meet the future mother of his children?" and the creators were adamant that it would happen in the Grand Finale. The thing is they weren't sure how long they would get to tell this story and there were several admitted plans in place in case they were or were not renewed. Victoria of season one was outright confirmed to be the Mother in case the initial 13-episode order was all they had. Stella is theorized to also be a back-up Mother (given how they meet ties directly into the implied "Myth Arc") if season three was the end. Once ratings stabilized it seemed that the writers had a clear idea of how the show would end by introducing some more solid clues (the 100th episode has Ted meeting the Mother's roommate and getting a lot of, still vague, information on her) but still had to keep things flexible because now they weren't sure when they were ending.