Alice Brady
Brady — not to be confused with Alice from "The Brady Bunch" — won best supporting actress for her role in "In Old Chicago" at the 1937 ceremony. Since Brady wasn’t there, a man came onstage and accepted the plaque on her behalf (supporting actors didn’t get a statue until 1944). Turns out the man was an impostor, and actually stole the plaque. He was never found and neither was the plaque, but the Academy gave Brady a replacement.