Continuity mistake: When Dorothy finds the small green glass ornament, she shouts "Oz!", and the Scarecrow appears. In the shot when Dorothy calls "Oz", there are two large ornaments to the right side of the counter. Then when it cuts and the scarecrow "appears", the ornaments have now moved a few feet away, so that the Scarecrow can comfortably "appear."
Continuity mistake: When Dorothy and Billina are in the crate surrounded by water, then water begins to soak up, so that it gets very shallow. In the next shot over Dorothy's shoulder the water is suddenly more deep and rippling again. Also, the sand, once it dries up, goes from damp to drier.