The horses in Emerald City palace were colored with Jell-O crystals. The relevant scenes had to be shot quickly, before the horses started to lick it off.
Margaret Hamilton, who plays the Wicked Witch of the West, was a kindergarten teacher before becoming an actress.
The "oil" that was used to lubricate the Tin Man was not really oil. It was discovered that oil would not photograph well, so they used chocolate syrup instead.
The "tornado" was a thirty-five foot long muslin stocking, photographed with miniatures of a Kansas farm and fields.
In Frank L. Baum's books, Dorothy was actually about 11 years old. As Judy Garland was in her mid-teens when she was cast, MGM pulled out all the stops to make her look as young as possible. Already on a chicken soup-only diet and appetite suppressants (as she was a little too chubby for the studio's liking), MGM upped her dosage and had a corset made to flatten any signs of a womanly figure. Garland had to have special lessons on how to walk, talk and dance normally as the corset was so tight. The costume department dressed her in a childish pinafore dress and gave her little girl plaits too. Even with these efforts it's still clear to see that Garland looks older than 11 in the film.
According to lead Munchkin Jerry Maren, the "little people" on the set were paid $50 per week for a 6-day work week, while Toto received $125 per week.
The coat that Frank Morgan wears as Professor Marvel was bought second-hand for the film. It was only discovered later that it once belonged to L. Frank Baum, the creator of the Oz stories. The name sewn into the garment was shown to his widow, who confirmed that the coat did indeed once belong to the author.
Margaret Hamilton proved too authentic as the wicked witch as it was reported that mothers throughout the country had to carry their children out of movie theaters because the children were horrified by her make up.
Buddy Ebsen, the original actor hired to play the Tin Man, became very ill from the metallic makeup and was not able to appear in the movie, but his voice can still be heard singing "We're off to see the wizard," when Dorothy and her friends are dancing down the yellow brick road after the forest scene.
Some of the more minor mistakes in the film may have been left in because there was no time to shoot more accurate takes. This picture wasn't expected to be as big of a hit as it was, and when it went over-budget and took longer than expected to shoot, a lot of pressure was put on the crew and director to finish it. MGM wanted to put Judy Garland in a film with Mickey Rooney, as he was a bigger star than she was (and the studio thought starring with him would help her career), so the crew of the film had to rush to get it all shot and edited so Garland could be released. There was also pressure as every Technicolor camera in existence at the time was needed for Gone With the Wind, which had already started filming.
During filming, Toto was accidentally stepped on by one of the Witch's guards, and had to be replaced for several days with a look-alike. Other on-set accidents included two winged monkey actors who fell when their support wires snapped, and Margaret Hamilton being severely burned when the elevated platform that made her disappear from Munchkinland in a puff of smoke malfunctioned.
The steam shooting from the Tin Man's cap startles Toto, who runs out of the shot.
Originally, the Wicked Witch of the West recorded a lot more scenes. Most of these scenes were cut because the director thought it would scare the children too much. You can see evidence of this when the foursome are surrounded by her guards and the witch comes and says something like Ring around the Rosie and it cuts to a different scene with her in it.
I have to post this to refute the comment that denied the existence of an alternate ending. I was overjoyed to find a comment here from someone else who remembered seeing a different ending just one time in the 1960s. I've spent my whole life trying to find someone else who remembered this. In the 1960s the annual broadcast of the film had hosts. I, and two of my friends, ever since childhood always remembered that one year the movie had a different ending. I've always sensed it was the year that the hosts were Liza Minnelli and Lorna and Joey Luft. We never could remember what the different ending was, but we recalled that it was black and white and that our reaction was: It wasn't just a dream that time. Now that I've read this other person's memory of the camera's panning to the ruby slippers under the bed, in black and white, I remember that's what I saw. Another commenter says that there's no evidence that the scene ever existed. I am here to verify that someone else has never stopped wondering for over 40 years about a vague memory of a different ending from one airing in the 1960s.