Continuity mistake: When the Lion is singing about being King of the Forest, the Tin Man breaks a flower pot to "crown" him. Pay close attention, as the break marks on the pot change. In one shot there's a big jagged edge sticking up (obviously longer than the others) and in the next shot most of the edges are even.
Continuity mistake: When Dorothy first meets the scarecrow, there is a patch of grass in front of the field the scarecrow is in. On the grass there's two beds of flowers. The flowers change colors from when Dorothy meets the Scarecrow, to when the Scarecrow starts his "If only I had a Brain" song. (00:33:00 - 00:35:30)
Continuity mistake: At the witch's castle, the witch throws a sand timer to the floor from a banister, and it smashes in front of a large group of her guards. But in the next shot, when the candle chandelier lands on all the guards, you can see that the smashed timer has disappeared. (01:21:20)
Revealing mistake: In the scene where Dorothy, Tin man, Lion and Scarecrow are in the haunted forest and are discussing whether or not to believe in spooks, Tin Man is lifted up into the air and then comes crashing to the ground. You can see his pants wrinkle up as he's trying to get up. Since when does tin wrinkle like that? (01:14:00)