Dorothy: How can you talk if you haven't got a brain?
Scarecrow: I don't know. But some people without brains do an awful lot of talking, don't they?
In the beginning while Dorothy is still on the farm, she walks along the pig pen fence and then falls in. When Bert Lahr picks her up out of there her dress is perfectly clean.
Professor Marvel, the Cabbie, the doorman, the guard, and the wizard himself are all the same actor, Frank Morgan.