--- a/mercurial/bookmarks.py	Fri Feb 18 17:09:08 2011 -0600
+++ b/mercurial/bookmarks.py	Thu Feb 24 14:38:50 2011 +0100
@@ -127,8 +127,11 @@
     update = False
     mark = repo._bookmarkcurrent
     if mark and marks[mark] in parents:
-        marks[mark] = node
-        update = True
+        old = repo[marks[mark]]
+        new = repo[node]
+        if new in old.descendants():
+            marks[mark] = new.node()
+            update = True
     if update:
         write(repo)
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-issue1502.t	Thu Feb 24 14:38:50 2011 +0100
@@ -0,0 +1,42 @@
+http://mercurial.selenic.com/bts/issue1502
+
+Initialize repository
+
+  $ hg init foo
+  $ touch foo/a && hg -R foo commit -A -m "added a"
+  adding a
+
+  $ hg clone foo foo1
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo "bar" > foo1/a && hg -R foo1 commit -m "edit a in foo1"
+  $ echo "hi" > foo/a && hg -R foo commit -m "edited a foo"
+  $ hg -R foo1 pull -u
+  pulling from $TESTTMP/foo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  not updating, since new heads added
+  (run 'hg heads' to see heads, 'hg merge' to merge)
+
+  $ hg -R foo1 book branchy
+  $ hg -R foo1 book
+   * branchy                   1:e3e522925eff
+
+Pull. Bookmark should not jump to new head.
+
+  $ echo "there" >> foo/a && hg -R foo commit -m "edited a again"
+  $ hg -R foo1 pull
+  pulling from $TESTTMP/foo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+
+  $ hg -R foo1 book
+   * branchy                   1:e3e522925eff