Revealing mistake: When Elizabeths car is stolen, Elizabeth puts the phone down and runs across the street. The camera then pans into the phonebox, and reflected in the grey shiny surface of the phone is someone in a white shirt. (00:06:00)
Revealing mistake: When Fred gets his head stuck in the fridge you can see that his head isn't really stuck but merely bent down. Also, his hair goes in red but comes out brown.
Revealing mistake: In the scene where Drop Dead Fred and Elizabeth are pretending to be burglars you can see constantly throughout that young Elizabeth has fake long hair extensions in, this relates to later on in the film when Elizabeths hair gets cut off by Fred, but the fake hair extensions proves it was never that long to begin with.
Answer: When she trapped Fred in the box, she was closing off the part of her imagination. The part of the brain that stops believing in fairy tales and decides to grow up.