Other mistake: The Witch's castle is shown twice, the first time is in the beginning of the scene when Dorothy is held in her room, and the second time is when Toto brings the three back to rescue her. Each time the painting is almost exact, even down to the lights in the windows of the castle. The only thing that changes is the walk way up to the castle, which is entirely different each time. (01:20:10)????
Revealing mistake: Throughout the movie, you can tell that if the main characters walking into the distance, the "faraway" landscape is a giant matte painting. (00:33:35)
Continuity mistake: Just before the group starts skipping towards the Emerald city, there is snow everywhere, but in the next shot, it's completely disappeared.