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 pretty much how she was with NSYNC (and Lance Bass in particular, who voiced one of the band members in this episode).
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.
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: The Lamby Lamby Dance in the episode, "The Inconveniencing", has the same name as a certain toy lamb in Doc McStuffins, a Disney Junior show.
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."
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 basically 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, and Waddles is Cinnamon Bun.
Schedule Slip: For some unexplained reason there are a lot of long delays between episodes in the US.
Literally in the episode "Double Dipper" with Dipper and all his clones.
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.
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 (rumored to be Lauren Faust).