We can say with one hundred percent certainty that if this happened today it would end with a lawsuit. Judy Garland was only sixteen years old when she played the part, and at the beginning of her career. She was also full of the vibrancy and immaturity of youth, as we all were once. Director Victor Fleming didn't have any appreciation or time for this, though.
Garland was struggling to get through a scene with the cowardly lion whilst keeping a straight face, finding something funny every time and causing filming to be stopped while she recomposed herself. Fleming tired of this, pulling her aside and telling her she needed to take the process more seriously. To drive the message home, he slapped her in the face! Fleming was the fourth director appointed to the movie, which endured a troubled time of production, and had been briefed to straighten the movie out and get it finished.