Well, he DID infamously say "I am a jelly [doughnut]!" (Ich bin Berliner)
He said "Ich bin eine Berliner." It was, a few years later, MIStranslated in a book to "I'm a jelly doughnut." This was because the author believed that you don't use "ein" or "a" when you translated "I am a Berliner" into German from English.
"I am an American" would be "Ich bin Amerikaner" when properly thus translated.
But the author was actually wrong. Kennedy was not actually from Berlin and of course, was not German, and everyone knew it. What he was doing was using rhetoric, saying "I am with you." In German, using rhetoric, it actually IS correct to use the "ein" in the place Kennedy used it. No German there was surprised or annoyed, they knew what he said.
But that misunderstanding in that book "went viral," as we say now. It's become a urban legend of sorts.
But Kennedy actually did get it right.
EDIT: oooops, Ben beat me to it.
