This generates an incorrect fall-through when A is &quot;a&quot; and FOO returns NIL:<div><br></div><div><div>(defun blah (a)</div><div>  (case a</div><div>    (&quot;a&quot; (when (foo) (return-from blah 111)))</div><div>

    (&quot;b&quot; t)))</div></div><div><br></div><div>Note that if you replace WHEN with IF, it works.</div>