Continuity mistake: Prior to the Tin Man's dance after he's saved by Dorothy and the Scarecrow, there is a shot of Dorothy with the Scarecrow and the Scarecrow is holding the oil can. The camera turns to the dancing Tin Man and when it turns back to Dorothy and the Scarecrow, the oil can has somehow ended up in Dorothy's hand basket.
Continuity mistake: During most of the movie, all the characters have at least three shadows, and when the guy that guards the entrance to the Wizard first goes in to speak with him, he has at least 4 or 5.
Continuity mistake: During the Tin Man dance, Toto switches from one side of the Tin Man to the other side without walking there.
Continuity mistake: When the witch is making her way towards the house a shadow disappears and reappears.
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. (01:14:05 - 01:15:05)
Other mistake: The winds during the tornado were so strong that it ripped a tree clean from the ground but 2 seconds later proceeds to barely blow over whatever she set down. How is she not getting blown away?