David Madore posted the following puzzle to comp.lang.scheme in late 1999:
(let* ((yin ((lambda (foo) (newline) foo)
(call/cc (lambda (bar) bar))))
(yang ((lambda (foo) (write-char #\*) foo)
(call/cc (lambda (bar) bar)))))
(yin yang))
Spoiler: https://stackoverflow.com/q/2694679/23567