AFAICT, it's not a valid keyword here, so not a big deal.  But other invalid keywords are silently dropped, so probably something isn't being handled here.

** unknown exception encountered, please report by visiting
** https://mercurial-scm.org/wiki/BugTracker
** Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC v.1500 32 bit (Intel)]
** Mercurial Distributed SCM (version 4.5-rc)
** Extensions loaded: purge, schemes, strip, mq, blackbox, extdiff, rebase
Traceback (most recent call last):
  File "../hg", line 41, in <module>
    dispatch.run()
  File "e:\Projects\hg\mercurial\dispatch.py", line 88, in run
    status = (dispatch(req) or 0) & 255
  File "e:\Projects\hg\mercurial\dispatch.py", line 183, in dispatch
    ret = _runcatch(req)
  File "e:\Projects\hg\mercurial\dispatch.py", line 324, in _runcatch
    return _callcatch(ui, _runcatchfunc)
  File "e:\Projects\hg\mercurial\dispatch.py", line 332, in _callcatch
    return scmutil.callcatch(ui, func)
  File "e:\Projects\hg\mercurial\scmutil.py", line 154, in callcatch
    return func()
  File "e:\Projects\hg\mercurial\dispatch.py", line 314, in _runcatchfunc
    return _dispatch(req)
  File "e:\Projects\hg\mercurial\dispatch.py", line 918, in _dispatch
    cmdpats, cmdoptions)
  File "e:\Projects\hg\mercurial\dispatch.py", line 673, in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "e:\Projects\hg\mercurial\dispatch.py", line 926, in _runcommand
    return cmdfunc()
  File "e:\Projects\hg\mercurial\dispatch.py", line 915, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **strcmdopt)
  File "e:\Projects\hg\mercurial\util.py", line 1195, in check
    return func(*args, **kwargs)
  File "e:\Projects\hg\mercurial\util.py", line 1195, in check
    return func(*args, **kwargs)
  File "e:\Projects\hg\hgext\mq.py", line 3583, in mqcommand
    return orig(ui, repo, *args, **kwargs)
  File "e:\Projects\hg\mercurial\util.py", line 1195, in check
    return func(*args, **kwargs)
  File "e:\Projects\hg\mercurial\commands.py", line 2028, in files
    return cmdutil.files(ui, ctx, m, fm, fmt, opts.get('subrepos'))
  File "e:\Projects\hg\mercurial\cmdutil.py", line 2914, in files
    fm.startitem()
  File "e:\Projects\hg\mercurial\formatter.py", line 173, in startitem
    self._showitem()
  File "e:\Projects\hg\mercurial\formatter.py", line 379, in _showitem
    self._renderitem(self._tref, item)
  File "e:\Projects\hg\mercurial\formatter.py", line 398, in _renderitem
    self._out.write(templater.stringify(g))
  File "e:\Projects\hg\mercurial\templatefilters.py", line 372, in stringify
    return "".join([stringify(t) for t in thing if t is not None])
  File "e:\Projects\hg\mercurial\util.py", line 1017, in increasingchunks
    for chunk in source:
  File "e:\Projects\hg\mercurial\templater.py", line 1272, in _flatten
    for i in thing:
  File "e:\Projects\hg\mercurial\templater.py", line 413, in runtemplate
    yield evalrawexp(context, mapping, arg)
  File "e:\Projects\hg\mercurial\templater.py", line 311, in evalrawexp
    return func(context, mapping, data)
  File "e:\Projects\hg\mercurial\templater.py", line 404, in runsymbol
    return v(**pycompat.strkwargs(props))
  File "e:\Projects\hg\mercurial\templatekw.py", line 528, in showfiles
    return showlist('file', args['ctx'].files(), args)
AttributeError: 'NoneType' object has no attribute 'files'

(I thought the log command would filter {files} down based on a fileset provided, but it doesn't seem to when --rev is given.  So I thought maybe files was the command I remembered, and forgot to clean up the template.  But that would be a nice feature, to be able to walk every revision, and print out files over a certain size, for example.)