This one's not as subtle as the last, but I still think it's pretty amusing (note: the events in this excerpt from Against the Day take place around the time of World War I, several decades before John F. Kennedy).

"Ich bin ein Berliner!"

"Excuse me?" The patient seemed anxious to speak with Kit.

"He will not harm you," Dr. Dingkopf assured him as attendants adroitly steered the patient away. "He has come to believe that he is a certain well-known pastry of Berlin--similar to your own American, as you would say, Jelly-doughnut."

"How long's he been in here?"

Shrug. "A difficult case. The Jelly-doughnut being such a powerful metaphor for body and spirit, to find one's way to sanity merely through reason becomes quite problematic--so we must resort to Phenomenology, and accept the literal truth of his delusion--bringing him into Göttingen, to a certain Konditerei where he is all over powdered with Puderzucker and allowed to sit, or actually recline, up on a shelf ordinarily reserved for the pastries. When he starts in with his 'Ich bin ein Berliner,' most customers try only to correct his diction, as if he is from Berlin and has meant to say 'Ich bin Berliner'--though sometimes he is actually purchased--'Did you want a bag for that, madame?' 'Oh, no, no thank you, I'll eat it right here if I may.'"

"Well--if that doesn't bring him back to reality..."

"Ach, but no, he only remains inert, even when they attempt to...bite into--"