Lance Bass knows a thing or two about being in boy bands. One of the clones even does *NSYNC's signature "Bye Bye Bye" move.
Alex Hirsch's sister Ariel has a voice cameo in "Boyz Crazy," as a crazy fan of Sev'ral Timez, which, according to Alex, was how she was with NSYNC (and Lance Bass in particular, who voiced one of the band members in this episode).
In Blendin's Game, Blendin Blandin tells Lolph and Dundgren that he will continue stammering until the twins are found. The voice actor, Justin Roiland, voices the title characters of Rick and Morty, two characters that stammer a lot.
Stan has Book 1 and he is aware of the crazy supernatural stuff that happens in town but lies about it.
Wendy knows about Dipper's crush on her.
Soos becomes a zombie in "Scaryoke".
The author of the books has 6 fingers.
Old Man McGucket was intimately involved with the mysteries of Gravity Falls before he went crazy — though he turned out not to be the author of the journals, as everyone assumed, but merely the true author's lab assistant.
Jossed: Before Bill Cipher was revealed as a character, some fans guessed that the triangle at the end of the theme song was just there for show and would have no in-show significance. Oh, how wrong were they.
Old Man McGucket is not the author of the journals.
Man of a Thousand Voices: Alex Hirsch voices at least five different characters in the show, and they all have their own distinct voice.
More specifically, he voices: Stan, Soos, the gnomes, Bill Cipher, Quentin Trembley, Old Man McGucket, Norman, Paper Jam Dipper, and additional background characters.
Grey DeLisle also voices many incidental characters on the show, some of the most notable being Mrs. Gleeful in "Little Dipper," the Summerween Superstore clerk and Gorney in "Summerween," Grunkle Stan's new voice in "Bottomless Pit," and the woman who asks Soos for directions and ends up falling in love with him in "Carpet Diem."
Name's the Same: Same company example: The Lamby Lamby Dance in the episode, "The Inconveniencing", shares its name with a character on Disney Junior's Doc McStuffins.
Old Shame: Alex Hirsch is, according to a reddit thread, not terribly fond of the 11-minute pilot used to pitch the show.
"Any chance we'll see the full unaired pilot at some point in the future?"
"Oh lord I hope not. That thing was a mess, haha! That would be like showing you awkward photos from my high school prom. We all have dark horrible secrets. That 11 minute Flash pilot was mine."
Also, certain tropes in this site are sometimes dropped in episodes and on staff comments.
The Other Darrin: Strangely enough, in a commercial for Gravity Falls mealbags at Subway, Kristen Schaal does not voice Mabel. Even stranger, all of the dialogue for the twins in that CM is taken from clips of previous episodes, and they didn't dub over Jason Ritter's voice for Dipper. Maybe Kristen Schaal just doesn't like Subway.
Nathan Fillion voices Preston Northwest in season 2 while another actor voiced him in "Irrational Treasure".
A good amount of the cast is a mini Adventure Time reunion: Candy is BMO and Lady Rainicorn, Mabel is Jake Jr., Blendin Blandin and Bobby Renzobbi are Lemongrab, Pacifica is Susan Strong, Tambry is Flame Princess, Manly Dan is Jake, the Summerween Trickster is Choose Goose, Waddles is Cinnamon Bun, and Bud Gleeful is Martin.
Schedule Slip: For some unexplained reason there are a lot of long delays between episodes in the US.
Urban Legend of Zelda: There is a picture circulating the Internet of the Slender Man watching Dipper and Mabel from the background in one of the episodes. This is not the case; viewing the episode itself yields no Slender Man.
There was going to be an episode based on the David Bowie film Labyrinth. Mabel would love the movie, but Dipper would hate it due to a fear of puppets. Dipper wouldn't admit to being afraid of the puppet; he would just call the movie dumb. Mabel would summon the Goblins to take Dipper away so she could watch the film. The Goblins would come out of the film take Dipper to the Labyrinth. The Goblin King would try to get Mabel to go through the labyrinth to find Dipper, but Mabel would instead want to spend time with the Goblin King. The episode would end with Dipper admitting his fear of puppets. The episode wasn't made because the studio execsdidn't think the audence would get the reference, but they still managed to sneak a Labyrinth reference into one of the "Mabel's Guide to Life" shorts (where Mabel, Grenda and Candy dress Soos up as the Goblin King for one of their "flash makeovers").
Wendy's last name is Corduroy. And her middle name is Blerble.
Dipper, Mabel and Stan's eyes are brown (Mabel's green eyes in "The Inconviniencing" are implied to be a result of overdosing on Smile Dip), Wendy's are green, and Li'l Gideon's are blue. As for Soos? "It's a mystery, dude!"
Write What You Know: Alex Hirsch based the series on the childhood summers he and his twin sister spent camping with their great-aunt.
Write Who You Know: Dipper is based on Alex Hirsch himself and each of the other main characters on people he knows. Mabel is based on Hirsch's twin sister Ariel, Grunkle Stan on their Grandpa Stan, Soos on Hirsch's one-time roommate Jesus and Wendy on a girl he had a crush on in his youth.