Other mistake: As the Wizard is pulling his curtain closed, he says the famous, "Pay no attention to that man behind the curtain," in a loud, booming voice. But he's turned 180 degrees away from his microphone, which we see a second later needs to be right in front of his mouth to make the booming voice effect. (01:28:40)
Factual error: Tin doesn't rust.
Continuity mistake: In the farm, after Hunk the farmhand has had his finger smashed, he scolds Dorothy. When she starts to leave, he hammers his hand and takes off his glove, shouting and turning around in pain. The angle swaps to a wide angle and he is working totally calmly with both gloves on.