Visible crew/equipment: When Dorothy is strapped down to the pedestal by the doctors, they then proceed to push her down the hallway, and pass the camera on their way. As they pass, the camera casts a clear shadow on the sheet covering Dorothy.
Visible crew/equipment: When Dorothy falls down into the Nome Kings lair, she falls to the floor and slides along into the scarecrow. The black sled she is sliding on is momentarily visible as she passes the camera. (01:07:05)
Visible crew/equipment: When Dorothy is speaking to Aunt Em about the key she found, Aunt Em says "That legs mended, it's mended..." then she gets up and passes the camera, which casts a clear shadow on her as she walks past it.
Visible crew/equipment: When Billina and Dorothy are stuck in the crate, when Billina says "how's my grammar," you can see a wooden board has been nailed to the crate to cover up the spaces in between the bars, so that the puppeteer can crouch down and control Billina. (00:23:00)
Continuity mistake: After Dorothy winds Tik Tok up (after he had gone beserk) her pigtails alternate from what shoulder they hang over (ie. her right pigtail was hanging behind her shoulder in the first shot, but then hangs in front of it in the next shot.)
Visible crew/equipment: When Billina lands on the couch with the cord, as she flies down some mechanism controlling her can be seen, attached to her feet and legs. (00:50:50)
Continuity mistake: When The Nome King shows his ruby slippers to Dorothy, and tells her that he could send her back to Kansas, in frontal shots both his feet are down on the floor, but in shots from behind he is wiggling his right foot up in the air.
Revealing mistake: When Mombi's headless body awakens and sits up, in the first two shots it is easy to notice that her body is actually simply a puppet of some sort, as the motion is not very fluid and it looks like a mannequin. Once she stands up, she becomes an actress whose head has been removed with special effects.
Revealing mistake: When Dorothy steals the key from Mombi's cabinet, as Mombi's body picks up her head to put on, you can see a small error. As Mombi lifts up her head, you can see the bottom of the cabinet warp slightly for about two frames (slow motion helps but is not needed), revealing that the actress simply had her head through a hole in the bottom of the cabinet (to make it appear severed), and her shoulders bumped into the bottom, making it shake/warp for a second.
Visible crew/equipment: When Mombi leads Dorothy into the 'head' room, as the camera moves along a crewmember in a pink shirt, wearing glasses, is reflected in the glass screens of the head cases. (00:41:40)
Visible crew/equipment: Wires are clearly visible attached to Dorothy's shoulders, Jack Pumpkinhead's pumpkin head and his body, as everyone falls down from the sky after the couch goes bonkers. (01:02:05)
Continuity mistake: On The Nome King's mountain, everyone looks up, and Tik-Tok is standing closest to the edge, then the sofa-body of the Gump is there, then Dorothy on the other side of it. In the next shot Tik-Tok is suddenly beside Dorothy.
Continuity mistake: When the Wheelers have arrived back to Mombi's castle, in one shot you can see a broken pillar next to one Wheeler, totally bare. In the next overhead shot a long vine of ivy is laying on it. (01:09:20)
Visible crew/equipment: After Billina lays an egg in the Nome Kings mouth, the King drops Jack Pumpkinhead to the floor, and wires attached to him, out at the side, are visible to let him down gently. (01:28:55)
Continuity mistake: When Dorothy looks up and spots the head of the 'Gump' on the wall, it is covered with cobwebs. Later when Jack removes it for the couch, it is clean. (00:48:20 - 00:50:50)
Continuity mistake: When it shows the chickens on the farm, just before Toto runs over and scares them all, the position of the brood of chickens is completely different between shots, and the branches and logs around them have also changed around.
Visible crew/equipment: In shots of Dorothy and Ozma running away from the doctor's clinic, there is a noticeable outline of kneepads under their dresses, and as Ozma slips down the muddy bank, and into the river, her dress momentarily rides up, revealing it quickly.
Visible crew/equipment: As the camera is panning around to show off the variety of heads Mombi has collected, in one shot you can see the reflection of the cameraman with a camera in one of the glass screens. (00:41:50)
Continuity mistake: When the group lands on The Nome King's mountain, we see Tik Tok land with his body buried in the snow. Then in the next shot he is suddenly less buried than he was in the first shot. Then in the next shot he isn't even buried, and is just laying on a rock.
Continuity mistake: When Dorothy finds the small green glass ornament, she shouts "Oz!", and the Scarecrow appears. In the shot when Dorothy calls "Oz", there are two large ornaments to the right side of the counter. Then when it cuts and the scarecrow "appears", the ornaments have now moved a few feet away, so that the Scarecrow can comfortably "appear." (01:23:55)
Answer: She was in the human world so she was in disguise.