Revealing mistake: In the beginning when Mrs. Meers goes into the elevator, you can see that the elevator floor is a continuation of the room floor, just with a strip across it.
Revealing mistake: When Jimmy and Millie are sitting on the ledge of the building she works in, Jimmy leans forward, and the top of his head disappears - or actually fades into the background.
Revealing mistake: Jimmy and Millie are sitting on the ledge of the building and stand up. When Millie is about to fall and waves her arms the left one becomes semi transparent and merges with the background.
Revealing mistake: During all of Millie and Jimmy's flag pole scenes, and afterwards when they are sitting on the ledge, the people and cars on the street are motionless, revealing the matte painting.
Revealing mistake: During the flying scenes, the stagger wires of the plane flicker, giving the sense they appear and disappear because of the blue screen effect.