diff -r 40cc20042fb4 -r 8ae7626d8bf1 mercurial/subrepo.py
--- a/mercurial/subrepo.py	Wed Feb 29 14:41:11 2012 +0100
+++ b/mercurial/subrepo.py	Wed Feb 29 14:24:57 2012 +0100
@@ -500,7 +500,7 @@
         anc = dst.ancestor(cur)
 
         def mergefunc():
-            if anc == cur:
+            if anc == cur and dst.branch() == cur.branch():
                 self._repo.ui.debug("updating subrepo %s\n" % subrelpath(self))
                 hg.update(self._repo, state[1])
             elif anc == dst: