--- a/mercurial/commands.py	Sat Dec 01 12:54:34 2007 -0600
+++ b/mercurial/commands.py	Sat Dec 01 13:09:27 2007 -0600
@@ -194,6 +194,11 @@
     if op2 != nullid:
         raise util.Abort(_('outstanding uncommitted merge'))
     node = repo.lookup(rev)
+
+    a = repo.changelog.ancestor(op1, node)
+    if a != node:
+        raise util.Abort(_('cannot back out change on a different branch'))
+
     p1, p2 = repo.changelog.parents(node)
     if p1 == nullid:
         raise util.Abort(_('cannot back out a change with no parents'))
@@ -210,6 +215,7 @@
         if opts['parent']:
             raise util.Abort(_('cannot use --parent on non-merge changeset'))
         parent = p1
+
     hg.clean(repo, node, show_stats=False)
     revert_opts = opts.copy()
     revert_opts['date'] = None

--- a/tests/test-backout	Sat Dec 01 12:54:34 2007 -0600
+++ b/tests/test-backout	Sat Dec 01 13:09:27 2007 -0600
@@ -37,6 +37,22 @@
 hg backout -d '3 0' --merge tip
 cat a 2>/dev/null || echo cat: a: No such file or directory
 
+echo '# across branch'
+cd ..
+hg init branch
+cd branch
+echo a > a
+hg ci -Am0 -d '0 0'
+echo b > b
+hg ci -Am1 -d '0 0'
+hg co -C 0
+# should fail
+hg backout -d '0 0' 1
+echo c > c
+hg ci -Am2 -d '0 0'
+# should fail
+hg backout -d '0 0' 1
+
 echo '# backout with merge'
 cd ..
 hg init merge

--- a/tests/test-backout.out	Sat Dec 01 12:54:34 2007 -0600
+++ b/tests/test-backout.out	Sat Dec 01 13:09:27 2007 -0600
@@ -15,6 +15,13 @@
 removing a
 changeset 3:7f6d0f120113 backs out changeset 2:de31bdc76c0d
 cat: a: No such file or directory
+# across branch
+adding a
+adding b
+0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+abort: cannot back out change on a different branch
+adding c
+abort: cannot back out change on a different branch
 # backout with merge
 adding a
 reverting a