Revealing mistake: During the "Red" number in the Emerald City, a large screen is supposedly telecasting from the walking camera focused on the dancers below. Watch closely and you'll see the dancers' movements do not coincide with the movements displayed on the screen.
Revealing mistake: During the subway scene, when the first pillar breaks and starts to move towards Dorothy, the top of it is cardboard and wiggles back and forth as the pillar breaks. This is noticeable in the other pillars as well.
Revealing mistake: When Dorothy lands in the concrete sandbox, arriving in Munchkinland, behind her is the empty cutout placement of where one of the Munchkins will emerge from - before the Munchkins emerge from the walls.
