northrup@Topaz:~$ sbcl
    This is SBCL 1.5.8, an implementation of ANSI Common Lisp.
    More information about SBCL is available at <http://www.sbcl.org/>.
    
    SBCL is free software, provided as is, with absolutely no warranty.
    It is mostly in the public domain; some portions are provided under
    BSD-style licenses.  See the CREDITS and COPYING files in the
    distribution for more information.
    * (- 0.3 3/10)
    0.0
    * (= (- 0.3 3/10) 0)
    T
    * (= (- 3/10 0.3) 0)
    T
    * (= 0.3 3/10)
    NIL

So yeah, 0.3 and 3/10 are definitely distinct, but still apparently net out to exactly 0 nonetheless.