The other — the big, carved, brown wooden door at the far corner of the drawing room — was locked.
-- Narrator (I paragraph 46)

Importance: This locked door in the drawing room of Coraline and her parents’ new flat turns out to be a portal to a strange, reverse universe ruled by an evil woman who claims to be Coraline’s other mother.

The message is this. Don’t go through the door.’ He paused. ‘Does that mean anything to you?
-- The crazy old man from upstairs (II paragraph 31)

Importance: The crazy old man who lives in the upstairs apartment in Coraline’s house gives Coraline this message from the mice he claims he’s training for a circus. Although the message warns Coraline not to go through the door in the drawing room, she does not heed the warning.

You know, Caroline,’ she said, after a while, ‘you are in terrible danger.
-- Miss Spink (II paragraph 65)

Importance: Miss Spink reports to Coraline that she is in danger when she reads...