Several scenes had to be corrected 24 hours after the chapter's initial publishing: what coin Scrooge trapped Magica in, the names of Willow's fathers, and Nina's age when she stopped believing in the Titan (9, not 5).
The author's notes stated that Madame Mim is from The Sorceror's Apprentice, but she's actually from The Sword in the Stone.
Shrug of God: Gus G. was probably thinking about Flash Gordon (1980) when he made the reference, and he might've listened to the film's soundtrack at one point.
Throw It In!: When the chapter's mistakes were corrected, Waddles was also added to Dipper's first two scenes.
Sully going by Harvey was a mistake, but it's confirmed that "Harvey" is his first name and "Sullivan" is his middle name.
Gus G's nod to Flash Gordon was referencing the franchise as a whole, but he was probably thinking of the 1980 film when he did it.
Candy and Grenda were probably watching "Duck-tective", while Pacifica was sulking over Mabel leaving, hence why they didn't appear. Everybody had indeed been missing Mabel during everything.
Nora originating from Maine was due to the state being a hotspot for the supernatural.
Clara being a Big Time Rush fan was because it was the only popular band from the 2010's that JP knew of.
Nora already told Gus G. what really happened to Luke when he died while working for Department A.
Gus G. ended up keeping the thunder guitar after the battle royale.
