1.Zoey is seen with the same wishbone necklace Lily wore in Season 1. Perhaps she bought it at lilyandmarshallselltheirstuff.com, as she mentions having purchased Lily's paintings online.
2.This couple in the background who went from talking to getting engaged, to making a baby, to celebrating their kid's graduation, to the husband passing away, all in one scene.
3.In Barney's flashback to Moscow in 1807, he's drinking a White Russian, while James is drinking a Black Russian.
4.Sometimes How I Met Your Mother injects current-day ads into the background of old episodes — for example, this episode from December 2012 advertising the 2016 film Why Him?
5.In the episode where Marshall's father dies, there is a hidden countdown throughout the episode that leads up to the news.
6.Conan O'Brien was once an extra at MacLaren's Pub and didn't have a single line.
7.The clock in Lily and Marshall's apartment always reads 4:20.
8.The kid who played Ted's nephew in "How Lily Stole Christmas" also played Barney's fake son, Grant, in "The Stinsons."
9.In the episode, "Disaster Averted," Marshall uses some slaps on Barney, saying he'll save the last of 'em "for a rainy day." On "End of the Aisle," when he uses his final slap on Barney, it's raining.
10.The name of the play Barney puts on in the Season 2 episode "Stuff" is Suck It Lily.
11.The club Ted goes to on St. Patrick’s Day — when he regretfully behaves like a jerk — is called “Low Point,” a reference to the way he’s feeling at the time.
12.Even when attending a rave in Season 4's "Murtaugh," Barney still wore a tie.
13.Even Barney's Christmas tree is decorated with ties.
14.In the episode "Murtaugh," Marshall is coaching a children's basketball team. In one scene, one kid appears to blow or spit in the eye of another kid who tried to scoot him out of his seat.
15.The spaceship on Robin's old show Space Teens is a curling stone.
16.Season 9's episode "Mom and Dad" shows a Rastafarian hang glider "eating a sandwich."
17.This sign that says "DUK TAI" is a reference to Barney's duck tie.
18.The framed headline in a flash-forward to 2029 reveals that Marshall captured the Loch Ness Monster.
19.Robin can be spotted in the back of Tracy and Louis's photo.
20.All these women slept with Barney according to their baggage.
21.This grave that reads "Mother" may've been foreshadowing the series' ending.