According to Emily Perkins (young Beverly Marsh), the remaining Losers (Jonathan, Brandon, Adam, Seth, Ben, and Marlon) were in Jonathan's trailer, reading the script and the novel. When Emily entered to rehearse with them, they revealed to her that her character, Beverly, has sex with all the boys in the sewers within the novel, which resulted in Emily being embarrassed.
(at around 20 mins) In adult Ben's flashback to his first day at school, photos of Cujo (from Cujo (1983)) are visible on a bulletin board behind the teacher.
(at around 11 mins) In the scene with Pennywise (Tim Curry) and Georgie (Tony Dakota), Curry was so in character that Dakota remarked "Tim, you're scaring me." Curry responded with "Gee, I'm so sorry, but that's what I'm supposed to be doing. And you're supposed to be scared."
(at around 1h 13 mins) When he got Mike's phone call, Stan Uris and his wife were watching Perfect Strangers (1986), starring Bronson Pinchot. Pinchot would later be on the Stephen King series The Langoliers (1995).
(at around 41 mins) Beverly's father watches ABC Television, the station on which this miniseries first aired.