Final answer:
The incorrect "Dewey Defeats Truman" headline was published in 1948 by the Chicago Tribune due to early polling errors, leading to a premature announcement before Harry Truman was declared the winner of the presidential election.
Step-by-step explanation:
The year of the incorrect "Dewey Defeats Truman" newspaper headline was 1948. This headline was prematurely published by the Chicago Tribune on November 3, the day after the presidential election, before the final votes were tallied which showed that Harry Truman had actually won. The Chicago Tribune made the error due to relying on early poll results and not taking into account a late shift in voter opinion, which ultimately led to Truman's unexpected win with 49.5 percent of the popular vote and 57 percent of the Electoral College.