1.First, in Season 1 of How I Met Your Mother, Lily can be seen wearing a wishbone necklace. Then, in Season 6, Zoey is seen with the same necklace after admitting that she bought a bunch of Lily's stuff online.
2.In Game of Thrones, Daenerys mentions that braids represent the number of victories a person has won. As the series continues, you can see that she adds more and more braids.
3.In Parks and Rec, the drunk man Leslie sees on the playground is the same man who returns in the final episode to ask the parks department to fix a playground swing.
4.In Season 2 of Breaking Bad, Walt Jr. scolds Skyler for buying Raisin Bran, not Raisin Bran Crunch. Later on, you can see that she bought the correct type of cereal.
5.Also in Breaking Bad, the pair of pants that Walter loses in the desert during the pilot reappear when Walter passes them during the final season.
6.In Brooklyn Nine-Nine, Gina reveals that after Amy has had four drinks, she becomes more sexual. Later, when Jake and Amy go on their first date, they both order four shots and end up sleeping together.
7.In Season 2 of The Simpsons, Mr. Burns gives the Simpsons a giant statue of a head. Later, in Season 28, you can spot the statue in the basement.
8.In Season 1 of Adventure Time, Jake and Finn get hypnotized by King Worm. Later on, Jake and Finn realize they've been trapped in a dream world, and after they escape, you can see King Worm dying.
9.In Community, Abed tells Troy that he has experience delivering a baby. If you look closely, you can see Abed delivering a baby in the background of an earlier episode.
10.Also in Community, Beetlejuice is mentioned during Season 1 and Season 2. In Season 3, when Annie says his name again, you can spot him walking behind her.
11.In Season 2 of It's Always Sunny in Philadelphia, Charlie gives himself a tattoo that reads "badnew." In Season 6, you can see the faded tattoo on his arm.
12.In The Office, Jim receives a shamrock keychain during the office's Christmas yankee swap. Later on, you can see that Jim still has it.
13.Also in The Office, you can see that Michael is selling his smashed plasma screen TV at the office's garage sale.
14.In an episode of Broad City, Abbi spends way too much money on a dress. Throughout the rest of the show, Abbi wears the dress to every nice event she's invited to.
15.In Season 5 of Grey's Anatomy, Lexie tells Mark that when she's hurt, she likes to have her hair stroked — in Season 8, when Lexie is dying, Mark does just that.
16.In SpongeBob SquarePants, Squidward poorly draws a circle at his art show. Later on, he gets attacked by a sea bear because he still can't draw a perfect circle, which protects you from an attack.
17.In Stranger Things, Mike teaches Eleven how to tell time by reading each number out one-by-one. In Season 2, she still tells time that way.
18.In an episode of BoJack Horseman, Diane says that she accidentally Instagrammed a picture of the floor to BoJack's account. If you check the show's official Instagram account, you can see that they posted the photo months before Season 3 dropped on Netflix.
19.In Doctor Who, Clara finds a room filled with things the Doctor has kept over the years. If you look closely, you can see Amy Pond's model of the TARDIS.
20.In Season 1 of Rick and Morty, Morty makes a deal with Rick that he gets to choose every tenth adventure. In Season 3, Morty finally gets to choose an adventure.
21.And finally, in an episode of Gravity Falls, Grunkle Stan's mug, notebook, and pen get sucked into an open portal. Later, in a Rick and Morty episode, you can see those same objects fly out of a portal Rick opens.