Jess, in her dream, sees a lady that cut's her throat and says, 'Przepraszam'. The Polish word meaning 'I'm sorry'.
The majority, if not all of this production was shot in Wales in the UK and particularly the capital, Cardiff.
Don't Knock Twice shares its name with a video game - Don't Knock Twice (2017) which was released in 2017 for Microsoft Windows, PlayStation 4, Xbox One and Nintendo Switch and is loosely based on the same story.
When Jess destroys the door with an axe it seems to be a nod to "The Shining" (1980) since the perspective and the unusual camera movement are exactly the same.