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 both Let's Go to the Mall and Sandcastles in the Sand have been created and both songs are up for purchase on itunes. The Bro Code was also made into an actual book. Also, an in-universe example when Robin describes the building Ted designed as "a building in your head that became real."
Fake Nationality: 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.
Fan Nickname: Luke & Leia for Ted's kids, based off Ted being told to swear on his future children & responding "I swear on Luke & Leia."
Ascended Fanon: Season 9's "Unpause" reveals that Ted's son is called Luke, but his daughter is named Penny.
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.
In Season 9, when Ted and Robin do their private joke,note (salutes)Private Joke Lily points out, as many fans have, that you don't salute privates.
Gratuitous Foreign Language: In "The Best Man" comments on Shmosby videos are actually intelligible Finnish, but obviously created with a translation software.
Real Life Writes the Plot: 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." However, while filming the episode "Stuff" (in which Robin's five dogs are a major plot point), the scene in which Ted is kneeling down by the Dalmatian and petting its face caused Radnor to have such a severe allergic reaction that paramedics were called. 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)
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.
If the show hadn't gotten picked up for a second season, Victoria would have been the mother.
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.