Continuity mistake: After the talking trees throw their apples at Dorothy she walks up a small hill and finds the Tin Man. If you look behind her there's a house across the Yellow Brick Road. We just saw this exact same spot in the previous shot as they were being pelted with the apples and the house is not there. (00:40:20)
Continuity mistake: When the Wicked Witch tells Nikko "throw that basket in the river and drown him" the knife in front of the basket to the right of the hour glass is pointing right in the direction of the round chain hook embedded in the concrete underneath. The next time we see this knife it has moved 90 degrees in relation to the chain hook, without anyone touching it. (01:16:45 - 01:17:20)
Other mistake: When Dorthy's house is falling back to Kansas, just before it lands, there is about a 1 or 2 second shot of what looks like production notes written in blue ink.