An Imposter Steals A Statuette (1985)
Nicknamed "The Great Imposter," serial fame-seeking prankster Barry Bremen crashed the stage when Hill Street Blues star Betty Thomas was announced as the winner of Best Supporting Actress in a Drama, accepting the award on her behalf from stunned presenter Peter Graves even though she was indeed in the audience. (You can see him give his speech at the 7:50 mark here.) Bremen was later arrested and fined for theft, and Thomas was allowed to make her speech after a commercial break. Nice try, guy.