The Wizard of OzGoofs
The Mayor and his agents' voices get mismatched in at least two places. During the congratulatory music, one Munchkin interrupts the festivities and demands legal verification of the Witch's death. When they exchange words, saying "From now on you'll be history! You'll be hist - you'll be hist - you'll be history!", that "lawyer" Munchkin has a completely different voice, as though from a different actor. Furthermore, when the Mayor says that last "history!", we hear not his voice, but that of the "lawyer" Munchkin.
Boom mic visible
After Scarecrow is given an honorary diploma, he says "The sum of the square roots of any 2 sides of an isosceles triangle is equal to the square root of the remaining side". This is a misstatement of the Pythagorean Theorem, which is in fact about right triangles and not isosceles ones. However, this statement is not true about any triangle, and so it is completely wrong.
Crew or equipment visible
A flesh-colored hand is visible in the upper left-hand corner of the frame in the closeup of the Witch being splashed with water. The Scarecrow is wearing dark burlap gloves, the Winkies are wearing black gloves and Dorothy is holding the bucket, so the only explanation is that the hand belongs to a crew member throwing water on the Witch.
Errors in geography
The Twister seems to teleport between shots. this can be seen when the tornado is behind the house but then jumps miles away and changes position. This can also be seen when Dorothy is entering through the gate and tornado is about 50 meters away from the house but when we see the rest of the family closing the storm shelter, the tornado is now back where it was when Aunt Em was shouting for Dorothy.
The footage of the ruby slippers disappearing from the feet of the Witch of the East was originally projected on a process screen with the Witch of the West kneeling in front of the screen reaching for the slippers. This explains why the slippers are positioned at the far left of the shot, with the Witch of the West kneeling in front of the screen on the right. The film of the disappearing slippers was fuzzy and unconvincing when projected on the process screen so this scene was not used. This caused the continuity error of the Witch being inches from the slippers when she bends down to get them and the following shot where she is absent from the shot. In the current film there is footage remaining of the original shot with the process screen: when the Witch turns around and says "They're gone!". The process screen can be seen in this shot.
Incorrectly regarded as goofs
When Dorothy is in her bedroom at the beginning of the movie the windows are closed with the curtains on them. The twister blows 'both' windows out and the one hits her on the head causing her to pass out. In all the next shots of her in the bedroom, the right hand window with the curtain is back in the shot. It should not be there as it was blown out. However, this ignores that the movie presents the happenings in OZ as a dream Dorothy had after being knocked unconscious. Therefore, in her dream, the farmhouse is undamaged and not a goof at all. This could also explain many of the so-called goofs during telling of the OZ events.
Glinda the good witch asks Dorothy whether she is a good or a bad witch because the Munchkins are not sure but later in the conversation Glinda says that good witches are beautiful & bad witches are ugly. If this is the case then all witches can be identified as good or bad just by looking at them & Glinda would not have needed to ask Dorothy whether she was good or bad. The Munchkins would presumably have known this as well, not to mention the fact that only a good witch would have killed the wicked witch.