Continuity mistake: When the four are at the end of the hallway to see the Wizard, Toto is near the lion's feet. As it switches shots, he is behind The Tin Man.
Visible crew/equipment: Near the very beginning, when Dorothy is with Prof. Marvel, after she runs out of his trailer and the wind starts blowing, he follows her and goes towards his horse tied up outside. As he's walking towards the horse, you can see the shadow of a boom mic along his trailer. (00:14:55)
Revealing mistake: In the scene where the Lion sings his "If I Were King" song just before visiting the Wizard, if you watch closely above his moving tail, there is a string holding up and moving around the Lion's tail. (01:04:15)