I used a reference like :Class:`something` (note the capitalization) and
got this exception:

Traceback (most recent call last):
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/__init__.py",
line 135, in main
    app.builder.build_update()
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/builder.py", line
201, in build_update
    summary='targets for %d source files that are '
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/builder.py", line
241, in build
    self.write(docnames, updated_docnames, method)
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/builder.py", line
276, in write
    doctree = self.env.get_and_resolve_doctree(docname, self)
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/environment.py",
line 779, in get_and_resolve_doctree
    self.resolve_references(doctree, docname, builder)
  File
"/home/ianb/src/env/lib/python2.4/site-packages/sphinx/environment.py",
line 998, in resolve_references
    raise RuntimeError('unknown xfileref node encountered: %s' % node)
RuntimeError: unknown xfileref node encountered: <pending_xref classname
modname refcaption="False" reftarget="deliverance.rules.Drop"
reftype="Class"><literal
classes="xref">deliverance.rules.Drop</literal></pending_xref>