During the sequence where Dorothy meets the Scarecrow, Dorothy's pigtails were first short (above her shoulders) and as the song progresses her hair gets longer (below her shoulders), then short, and then long again.
When the witch scares the people in the town where Dorothy lands, she disappears into a cloud of smoke she creates. But you can clearly see her sneak down into a trap door below.
In the beginning, when the twister is coming to Dorothy's house and she is trying to get in to the cellar, on the first shot, there are three metal buildings in the background. On the next shot the fields have been harvested and the buildings are gone.
When the Wicked Witch first appears in the smoke, Dorothy has a lollipop in her hand. In the next shot, she doesn't have it anywhere near her.
Several times you can see the wire that makes the Lion's Tail wag.
During the Tin Man's dance, you can see a stagehand caught unaware as he dashes behind the chessy tree props as he tries to hide.
The characters change places from one shot to another as the wizard gives the tin man his heart, etc.
If you get one of the original copies of the movie, you can see a small figure behind the blue screen in the background climbing up something, then slip, and then swing across the background. Again it is quite small. It happens shortly after the wicked witch of the west throws a fireball at Dorothy and her pals.
The album "Dark side of the moon" by Pink Floyd seems to be in sync with the Wizard of Oz if you start it right after the lion roars the second time. Example: the song "brain damage" starts right when you see the scarecrow the first time and the first line is "the lunatic is on the grass".
When the wizard is getting ready to take of in the balloon, while most people watch Dorothy climb out of the basket and go after Toto, the Tin Man is unravelling the thing holding the hot air balloon down, then he "accidentally" lets go.
Right - final comment, and no arguments, because I've had more mails saying this than anything else. The object in the background which some people think is someone hanging themselves is in fact some kind of bird. Definitely.
You can clearly see that Toto is wired to the curtain that he pulls back to reveal the real wizard.