"Kids, let me tell you about the time that Carter Bays and Craig Thomas ruined a nine-year story arc."
For nine long, agonizing seasons, viewers awaited the reveal of how the love-lorn and affable Ted would, as the series title suggests, meet the mother of his two children. Each episode of How I Met Your Mother (HIMYM) with Ted (in 2030) narrating in the voice of Bob Saget in frame tale fashion to his two teenage children a story largely about his life before meeting their mother, a life spent with his friends in New York City (his best bud and lawyer, Marshall; teacher and artist Lily; womanizer Barney; and news-anchor and Canadian Robin).
The series creators, Carter Bays and Craig Thomas, served as executive producer throughout and wrote several of the 208 episodes. Only four directors ever directed the series, Pamela Fryman (196 Episodes) Rob Greenberg (7), Michael Shea (4) and Neil Patrick Harris (1). The series was nominated for a total of 28 Emmy awards and won 9, including Alyson Hannigan for Best Actress. The ninth and final season aired from September 23, 2013 until March 31, 2014 and its final episode, a two-part story entitled "Last Forever" watched by over 13 million viewers, is one of the most controversial series endings of all time.
The first episode of HIMYM opens with our narrator, Ted meeting reporter Robin for the first time and Marshall preparing to propose to Lily, his longtime girlfriend. While there's rampant chemistry between Robin and Ted, the narrator makes it a point to let the kids (and the audience) know that she is not the mother. In a later Entertainment Weekly article, Thomas and Bays state "Ted meets the perfect woman, and it's not his final love story." (Source)
Throughout the series, the showrunners Bay and Thomas would drop small hints as to the identity of the mother- an iconic yellow umbrella, that she was a roomate to a girl Ted once briefly dated, that she was his "student" a in an accidental Economics class, that the two had a shared love for The Unicorns and T.C. Boyle, and so on, which always left viewers on the precipice of knowing just enough to keep the quest for the mother interesting. And since the first episode of the series clearly told viewers that the mother was not Robin Sherbotsky, it was accepted that Ted and Robin's relationship was doomed from the start.
During the series second season, Bays and Thomas grew concerned that the two teenagers playing Ted's future children would age past the series run, so Turner and Bays elected to tape the series ending then, which in part locked them into an ending that made sense for the current story arc, which involved Ted (still) pining after Robin. They had no idea the series would stretch on for seven more seasons.
In the two part finale, the first episode focuses on the wedding of Robin and Barney, a wedding that the entire final season had revolved around-- from all of the guests arriving piecemeal to the quaint bed and breakfast by the sea, to debacles surrounding broken bottles of expensive, nearly irreplaceable scotch; a mythical golden locket once buried in the park that Ted goes to the end of the earth (literally) to find in order to reassure Robin that she is doing the right thing marrying Barney. All the while we're impatiently waiting to see who the mother is, as we have realized she is the bassist in the wedding band. Finally we see her, and we meet her on a subway platform. She's got the yellow umbrella. Life is magical. Cue the music. Cue the fireworks. TED HAS MET THE MOTHER.
Cue the credits? Nope. We find out during the second part that not only did Robin and Barney's relationship not survive their marriage, but the mother had died of some undisclosed ailment, off screen, 6 years BEFORE Ted tells his kids the story of how he met their mother.
The story doesn't end there. His daughter, Penny, doesn't believe it. She says, "No, I don't buy it. That is not the reason you made us listen to this. This is the story of how you're in love with Aunt Robin."
The night of the finale, the internet nearly broke with the reactions from fans. Tweets filled with rage or acceptance, from the fans that felt cheated by the ending (after all, they were led to believe the Ted/Robin storyline was nothing but folly), or fans that were longtime lovers of Ted and Robin, and wanted to see them together. And in the end, they did. We see a gray-haired Ted win his children's approval to go and date their Aunt Robin, who they CLEARLY adore. He shows up on her doorstep with a blue french horn, a nod to the pilot episode where he had done the same, and we see that Robin, who has traveled the world and never wanted kids and wanted to make a name for herself, and now is finally ready for her happily ever after with Ted Mosby.
Fan disappointment was so rampant and so violent that the series creators created an alternate series finale for the DVD series, (see here) one where Ted and the mother meet on a train platform after Robin and Barney's wedding in the rain; Ted sees the yellow umbrella and knows that it's her-- it's the one woman he's been waiting for- and the two share a conversation before a train rolls past and a voice-over concludes with "and that's how I met your mother." But even that didn't appease fans, since after all, the series they had loved and watched for nine years had always had the intention of the mother dying and Ted ending up with Robin. It wasn't a casual miscalculation of the series creators, it was their intent. And that, somehow, makes it even harder.
Critical Responses to the ending: http://www.rottentomatoes.com/m/tv_editorial/news/1930029/himym_tomatometer_watch_series_finale_currently_rotten/?hub=15