On the first day she was to film the hypnosis scene in the town square, Judy Garland was severely disoriented from her various medications that she began to panic at the sight of the lit torches on the set and hallucinate that she was on fire. She began asking extras and crew members if they had any Benzedrine or reefers, and eventually was taken home and put to bed, still in her costume.
According to Judy Garland's biographer, Gerold Frank, The Pirate (1948) was the first film ever in which the studio hired a psychiatrist, paid for out of the film's production budget, to treat the star during shooting and make sure she was mentally healthy enough to perform.
While one dance sequence was being rehearsed, Harold Nicholas was just going through the motions, and Gene Kelly accused him of not knowing the routine - so Nicholas danced the whole routine, alone, full-out and flawlessly. Kelly was speechless.
The torrid romance enacted by Judy Garland and Gene Kelly in the song-and-dance number "Voodoo" so enraged MGM chief Louis B. Mayer that he demanded the negative be burned.