diff -r b8801b58bbd8 -r 58e040c51231 mercurial/dirstate.py
--- a/mercurial/dirstate.py	Sun Feb 07 11:38:36 2010 +0100
+++ b/mercurial/dirstate.py	Thu Feb 11 12:02:48 2010 -0200
@@ -202,6 +202,8 @@
         self._pl = p1, p2
 
     def setbranch(self, branch):
+        if branch in ['tip', '.', 'null']:
+            raise util.Abort(_('the name \'%s\' is reserved') % branch)
         self._branch = branch
         self._opener("branch", "w").write(branch + '\n')