When Kitty is telling the other ghosts about the Highams' party, Pat asks whether they had a buffet, and mentions pineapple, to which Kitty replies that the Highams were wealthy, but not royalty. This is both a reference to Kitty's past in Restoration England, when pineapples were so expensive they were often used only for display, and the series Horrible Histories, that shares most of the cast and where this particular subject was explained.
Thomas discloses his date of death: October 10, 1824.
First appearance of Puritan ghost Annie.
Jim Howick (Pat Butcher) and Holli Dempsey (Isabelle) have both appeared in the TV show, The Aliens (2016), playing Dominic and Holly respectively.
Robin appears to be communicating with the horses ridden to Button House. Robin also communicates with Barclays dogs in S2E6, Bump in the Night.