"I am a jelly doughnut!"
June 26, 1963 statement by JFK apparently intended to reassure citizens of West Berlin of the unwavering power and commitment of their ally, the United States. In context, the President had said:
"All free men, wherever they may live, are citizens of Berlin, and, therefore, as a free man, I take pride in the words 'Ich bin ein Berliner.'"
According to armchair linguists, the President should have left out the "ein" and said simply "Ich bin Berliner". That is, assuming he really had intended to declare himself symbolically a citizen of Berlin.
Unfortunately, it's not true. Due to some confusing German rules about the use of indefinite articles, Kennedy's statement would have been acceptable with or without "ein". Certainly, the Berliners attending his speech seem to have understood his meaning, and roared their approval.
There's an overview of the purported gaffe, along with audio and video clips, at http://www.serve.com/shea/jfkberl.htm