When Sophie grabs her notebook to go to school, you can see a note that reads "Watch Scorned Waifu" which started pre-production during the production of Sophie.
There are references to the director's first film, Sophie and the Serial Killers, seen and heard through out the film. The arcade game Luna and Nathan play is titled "Sophie and the Zombies" and audio from the beach scene can be heard. Later in the film, a storage unit customer tells Lindsey a story about the Drowning Lady. The actress playing Lindsey is the same actress who played Drowning Lady in Sophie and the Serial Killers.