Continuity mistake: After making the mud-pie Fred jumps into his box. In the closeup as he lands in it and Elizabeth closes it, you can see Elizabeth has pink trainers on. In the previous and following shots she has white socks on. (01:02:50)
Visible crew/equipment: When the nurse pushes Elizabeth into her bedroom and says 'Good night, flake' Elizabeth is standing near a picture hanging on the wall. The reflection of a cameramans arm holding a boom mic is clearly visible in it. (01:07:10)
Continuity mistake: When Lizzie listens to Charles on the phone to Anabella, she drops her prescription bottle. Then when she falls into Fred and they disappear, the prescription bottle has gone. After Lizzie comes back after the surreal journey, the prescription bottle is still there on the carpet. (01:20:30 - 01:27:05)
Audio problem: When Drop Dead Fred is hanging from the yellow rope outside the boat, and there is water spraying up, Fred says something like : "I dunno" and laughs. Notice this doesn't match his mouth movements.
Visible crew/equipment: This is the bit when Elizabeth's mother is taping up the Drop Dead Fred Box. Then when Elizabeth gets up from the stairs, walking past the round, marble table, notice the tape marker on the floor. Video Only.
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.
Visible crew/equipment: When Charles is taking the woman for a test drive in the car, Elizabeth pulls up next to them, and the boom mic is reflected in her door before it moves out of shot, though the boom pole is visible throughout the scene. (Only noticeable on DVD).
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.