Revealing mistake: During the scene near the end, at the climax of the film, when Ed is holding onto Janet for dear life outside her bedroom window, as Lorraine rushes over and pulls them back into the house, 'Janet' is a dummy.
Revealing mistake: When the girl is hanging from the ceiling her hair is following what gravity normally does. This gives away that the set is actually upside down and she is really on the floor. If she were stuck to the ceiling her hair would've been down on her face.
Answer: Valak was portrayed as a particularly powerful demon. In horror films demons are often portrayed as overconfident, and in this case it was thinking that Lorraine Warren would not be powerful enough to be able to defeat it. In addition, Valak revealed their name while Lorraine was in a dreamlike trance state, so it may have believed that she might not remember or make the connection at any point and be able to use it to her advantage.
dewinela