Although Harold Ramis passed away in 2014 and thus could not make a cameo alongside his fellow castmates, there is a bust of Ramis' head just outside of Erin's university office near the beginning of the film. The bust was later donated to the Harold Ramis Film School at Chicago's Second City, where Ramis began his career.
At one point Erin proclaims: "Books can't fly, and neither can babies." These happen in S.O.S. fantômes (1984) and S.O.S. fantômes II (1989), respectively.
Chris Hemsworth was concerned having to face off against three women for the interview scene, as he doesn't consider himself much of an improviser, but when the time came, he improvised the bit with the lens-free glasses and the Mike Hat line, among others.
After Erin comes to Abby's lab and announces that she has been fired, Abby says, "Unlike Columbia, this institute is 100% behind us." This is a reference to S.O.S. fantômes (1984), where Columbia University allowed its facilities to be used for filming, as long as its name was never mentioned.
Daniel Ramis: son of Harold Ramis has a small role as a Metal Head outside the concert Rowan attends.
Dominik Garcia: as Mayor Bradley's daughter, sitting with him when Erin barges into the Lotus Leaf warning him to evacuate the city.
Paul Feig: the voice on the other end of the walkie talkie that tells Rowan to go clean the bathrooms. He also appears to reference the original S.O.S. fantômes (1984), referring to a plumbing problem as "biblically bad."