Continuity mistake: In the scene where the Wicked Witch in Munchkinland goes over to observe the ruby slippers on her dead sister's feet, when she is up close there's not much space between her and the slippers but when she turns around and says "they're gone," she's further away, then she's closer again in the next shot.
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.
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.