This bug comes up if you try to do something like (return (cond... A quick workaround to this problem if you're using SBCL or Allegro is to (import-macros-from-lisp 'cond), since their cond works by expanding into nested if forms (this is what we use in our ParenScript application, which is why this bug slipped through the cracks for so long). The nested-if approach is also how I'm going to fix this problem and generate cond-expressions (yeah, this is another one of those bug reports that I use as a to-do).
Happy hacking, Vladimir