These 1960s citations seem to indicate earlier use, but I haven't found that yet.
5 November 1967, New York Times, pg. SM16:
Cleveland used to kid itself into believing its slogan, "Best Location in the Nation." Negroes preferred "Mistake on the Lake.
22 October 1968, New York Times, pg. 1:
Cleveland, that ethnically Balkanized "mistake by the lake" that elected a Negro Mayor last year, produced the largest Negro insurrection (as opposed to riot) in recent history.
Big Plum, Mistake On The Lake (Cleveland nicknames) • (0) Comments • Friday, March 04, 2005 • Permalink