STR (works on 4.2):

  $ hg init repo
  $ cd repo
  $ echo 0 > a
  $ hg ci -qAm 0
  $ printf 1 >> a
  $ hg ci -qAm 1
  $ cat a
  0
  1 (no-eol)

  $ hg revert -ir.^ --config ui.interactive=1 <<EOF
  > y
  > y
  > EOF
  reverting a
  diff --git a/a b/a
  1 hunks, 1 lines changed
  examine changes to 'a'? [Ynesfdaq?] y
  
  @@ -1,1 +1,2 @@
   0
  +1
  \ No newline at end of file
  revert this change to 'a'? [Ynesfdaq?] y
  
  $ cat a
  0

  $ cd ..


On 4.3-rc:

+  Traceback (most recent call last):
+    File "hg", line 45, in <module>
+      mercurial.dispatch.run()
+    File "mercurial/dispatch.py", line 86, in run
+      status = (dispatch(req) or 0) & 255
+    File "mercurial/dispatch.py", line 167, in dispatch
+      ret = _runcatch(req)
+    File "mercurial/dispatch.py", line 307, in _runcatch
+      return _callcatch(ui, _runcatchfunc)
+    File "mercurial/dispatch.py", line 315, in _callcatch
+      return scmutil.callcatch(ui, func)
+    File "mercurial/scmutil.py", line 145, in callcatch
+      return func()
+    File "mercurial/dispatch.py", line 297, in _runcatchfunc
+      return _dispatch(req)
+    File "mercurial/dispatch.py", line 927, in _dispatch
+      cmdpats, cmdoptions)
+    File "mercurial/dispatch.py", line 663, in runcommand
+      ret = _runcommand(ui, options, cmd, d)
+    File "mercurial/dispatch.py", line 935, in _runcommand
+      return cmdfunc()
+    File "mercurial/dispatch.py", line 924, in <lambda>
+      d = lambda: util.checksignature(func)(ui, *args, **strcmdopt)
+    File "mercurial/util.py", line 1056, in check
+      return func(*args, **kwargs)
+    File "mercurial/commands.py", line 4520, in revert
+      return cmdutil.revert(ui, repo, ctx, (parent, p2), *pats, **opts)
+    File "mercurial/cmdutil.py", line 3332, in revert
+      _performrevert(repo, parents, ctx, actions, interactive, tobackup)
+    File "mercurial/cmdutil.py", line 3434, in _performrevert
+      chunks = patch.reversehunks(chunks)
+    File "mercurial/patch.py", line 1526, in reversehunks
+      c = c.reversehunk()
+    File "mercurial/patch.py", line 970, in reversehunk
+      hunk = ['%s%s' % (m[l[0]], l[1:]) for l in self.hunk]
+  KeyError: '\\'
+  [1]

The first bad revision is:

changeset:   43372:66117dae87f9
user:        Jun Wu <quark@fb.com>
date:        Tue Jun 20 23:22:38 2017 -0700
summary:     patch: rewrite reversehunks (issue5337)