A Berliner is a sweet pastry, commonly referred to as a doughnut (donut in US English) in German speaking countries.
In 1963, John F. Kennedy, while giving a speech in West Berlin to express his support at the courage of its citizens in the face of communism, he gave a speech at the Brandenburg gate. He ended the speech with "I am proud to say, Ich bin ein Berliner". Unfortunately, in German, Berliner refers to the doughnut and hence the president's remark translated to "I am a doughnut". Nevertheless, the crowd still gave him a rapturous applause.