diff -r 39ea540dc154 -r e078ea9b4ce4 mercurial/commands.py
--- a/mercurial/commands.py	Thu May 23 17:53:19 2013 -0500
+++ b/mercurial/commands.py	Thu May 30 22:32:10 2013 +0100
@@ -1315,6 +1315,10 @@
         # Let --subrepos on the command line override config setting.
         ui.setconfig('ui', 'commitsubrepos', True)
 
+    if repo.vfs.exists('graftstate'):
+        raise util.Abort(_('cannot commit an interrupted graft operation'),
+                         hint=_('use "hg graft -c" to continue graft'))
+
     extra = {}
     if opts.get('close_branch'):
         extra['close'] = 1