"Best Location in the Nation" was soon ridiculed with "Mistake by/on the Lake." This nickname was popular in CB slang in the 1970s, during Cleveland's fiscal crisis.

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.