Continuity mistake: When the Tin Man is levitated and falls to the ground the metal covering his left leg is crumpled up. When we see him again from behind as the flying monkeys attack, the metal covering his leg has never been bent. The metal on his lower back has also been repaired since the fall.
Revealing mistake: When the doorman initially refuses to let Dorothy and friends into the Emerald City, they protest, saying the Good Witch of the North sent her; they show him the ruby slippers as proof. As the camera pans down to the slippers, look at the yellow brick road Dorothy is standing on - it is flat and looks as though it was simply painted.