--- a/mercurial/obsolete.py	Thu Aug 14 14:57:03 2014 -0700
+++ b/mercurial/obsolete.py	Thu Aug 14 14:59:42 2014 -0700
@@ -277,6 +277,8 @@
         for succ in succs:
             if len(succ) != 20:
                 raise ValueError(succ)
+        if prec in succs:
+            raise ValueError(_('in-marker cycle with %s') % node.hex(prec))
         marker = (str(prec), tuple(succs), int(flag), encodemeta(metadata))
         return bool(self.add(transaction, [marker]))
 

--- a/tests/test-obsolete.t	Thu Aug 14 14:57:03 2014 -0700
+++ b/tests/test-obsolete.t	Thu Aug 14 14:59:42 2014 -0700
@@ -64,6 +64,14 @@
   date:        Thu Jan 01 00:00:00 1970 +0000
   
   $ hg up --hidden tip --quiet
+
+Killing a single changeset with itself should fail
+(simple local safeguard)
+
+  $ hg debugobsolete `getid kill_me` `getid kill_me`
+  abort: bad obsmarker input: in-marker cycle with 97b7c2d76b1845ed3eb988cd612611e72406cef0
+  [255]
+
   $ cd ..
 
 Killing a single changeset with replacement