According to the near-ubiquitous urban legend, when Kennedy stood in front of the Rathaus Schöneberg on June 26, 1963, to express his solidarity with the people of West Berlin, he should have said "Ich bin Berliner," not "Ich bin ein Berliner." By including the indefinite article "ein," he likened himself to a Berliner Pfannkuchen, a type of jam-filled pastry. Somehow this blunder was missed by Robert Lochner, Kennedy's Berlin-raised translator who provided him with the phrase, as well as by the half-million native German speakers who cheered wildly upon hearing it.
This story is bogus for three reasons:
Grammar: As University of Wisconsin, Madison, linguist Jürgen Eichhoff noted in 1993, if Kennedy had said, "Ich bin Berliner," he would have been stating that he was literally from Berlin. The "ein" is gramatically necessary to make it clear that one is speaking in the figurative sense.
Regional cuisine: In other parts of the country, "Berliner Pfannkuchen," is indeed shortened to just "Berliner," but in Berlin, they just call it a "Pfannkuchen," in the same way that a Philadelphian would just say "cheese steak."
Context: Two days after the 9/11 attacks, Le Monde ran an editorial headlined "Nous sommes tous Américains," and nobody took the headline to read "We are all steak tartare." Because that would have been really dumb.