Buddy Ebsen
Ebsen was originally cast to play the Scarecrow in The Wizard of Oz.  However, Ray Bolger, who was playing the Tin Man, convinced Ebsen to trade roles for a rehearsal.  Ebsen had a very bad allergic reaction to the aluminum dust in the Tin Man's makeup.  He had to go to the hospital.  He was so sick that Jack Haley replaced him as the Scarecrow.