The actor originally cast as the Tin Woodman but forced to leave the role after a severe reaction to the makeup was Buddy Ebsen.
Ebsen was initially hired to play the Scarecrow, while Ray Bolger was cast as the Tin Woodman. Bolger wanted the Scarecrow role, so the two actors exchanged parts. Ebsen then wore makeup containing aluminum powder, intended to create the character’s metallic appearance.
The powder caused Ebsen serious respiratory distress, and he was hospitalized. “Aluminum poisoning” is the familiar quiz description, although modern accounts more precisely describe a dangerous reaction to inhaled aluminum-based makeup rather than ordinary poisoning. Ebsen’s illness became severe enough that MGM replaced him before filming his scenes was completed.
Jack Haley took over as the Tin Woodman, and his makeup was changed to an aluminum paste. Ray Bolger went on to play the Scarecrow, his most famous film role, while Ebsen later became widely known for television roles including Jed Clampett in The Beverly Hillbillies.