Judy Garland couldn't stop giggling while filming the scene in which Dorothy slaps the Cowardly Lion. So the director, Victor Fleming, took her aside, gave her a quick lecture then slapped her. She returned to the set and filmed the scene in one take, even though she still had a major problem keeping a straight face while watching Bert Lahr's crying bit. She can be seen stifling a smile, and she had to bury her face into her dog Toto at one point to hide her giggles (Pictured Above).
Victor Fleming was afraid that his slap would damage his relationship with Judy Garland and even told a co-worker he wished that someone would hit him because of how bad he felt, but Judy Garland overheard the conversation and gave him a kiss on the nose to show that she bore no hard feelings.
It's obvious that Judy Garland had a fun time performing as Dorothy in The Wizard of Oz, and I guess most young girls would laugh too, at the funny and wonderful Bert Lahr playing the role as the Cowardly Lion.