I've found dozens of incorrect birthdates on Wikipedia and in celebrity press releases. So far the one that's the furthest from the truth is the age of Efren Ramirez who played the teen-aged Pedro in Napoleon Dynamite. 1983 seems to be the birth year that everyone has.
California birth records for twins Efrain and Carlos Ramirez note that they were born in Los Angeles on October 2, 1973. Ten years earlier!!!