Continuity mistake: At the beginning of the movie, Oz gives his new assistant a gift, and leans in for a kiss. Both of them are seen to tilt their heads to their right for the kiss. The scene cuts to a wider shot, just before Frank barges in, and their heads are tilted to the left.
Continuity mistake: After Oz and Theodora have climbed down from the small cave where they hid from their first encounter with a flying baboon and are having a brief conversation, the brim of Theodora's hat alternates from being held up on her right side in the shots from behind her, and being down as it normally is in the shots from in front of her. Presumably this was done to prevent the hat brim from obscuring Oz's face.
Continuity mistake: Throughout the film, Glinda's hair continuously changes from being curled to straightened to curled again. This occurs often, and on occasion within the course of a single scene (for example when she introduces Oscar to the residents of Oz).
Continuity mistake: After Oz tells Finley he'll get a nice big pile of bananas, Finley tosses down Oz's bag and goes on a rant about monkey stereotypes. When he first throws down the bag, he is close to the middle of the yellow brick road. It cuts to a close up and he is suddenly standing right next to the edge of the road. (00:46:25)
Revealing mistake: When Oz first meets the Munchkins, some of them begin flipping down some stairs. The animation of this is very jerky and noticeable. (01:20:30)
Continuity mistake: Right after the Chinatown Massacre, when the trio of Oz, the Doll, and Wesley exit from the town, they are by the directional sign. At first, there are three places to go to. In the next shot, the middle one is missing.
Chosen answer: I believe you are taking a very modern day perspective. In our era of technology, scientific advancement, and general skepticism about everything, we are pretty jaded and cynical about things like magic and paranormal phenomena. At that time and place, audiences were far more willing to accept the possibility of true magic and sorcery, and weren't as prone to disbelieving what their eyes tell them. A "con" (short for "confidence") man is one who attempts to gain the trust of another, subsequently using deception, fraud and/or trickery for their own personal gain. I don't think it's unfair to label Oz this way, particularly at the beginning of the film.
Michael Albert