diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -968,6 +968,11 @@ def bookmark(ui, repo, *names, **opts):
             and not force):
             raise error.Abort(
                 _("a bookmark cannot have the name of an existing branch"))
+        if len(mark) > 3 and mark in repo and not force:
+            repo.ui.warn(
+                _("bookmark %s matches a changeset hash\n"
+                  "(did you leave a -r out of an 'hg bookmark' command?)\n") %
+                mark)
 
     if delete and rename:
         raise error.Abort(_("--delete and --rename are incompatible"))
diff --git a/tests/test-bookmarks.t b/tests/test-bookmarks.t
--- a/tests/test-bookmarks.t
+++ b/tests/test-bookmarks.t
@@ -311,6 +311,15 @@ bookmark with integer name
   abort: cannot use an integer as a name
   [255]
 
+bookmark with a name that matches a node id
+  $ hg bookmark 925d80f479bb db815d6d32e6
+  bookmark 925d80f479bb matches a changeset hash
+  (did you leave a -r out of an 'hg bookmark' command?)
+  bookmark db815d6d32e6 matches a changeset hash
+  (did you leave a -r out of an 'hg bookmark' command?)
+  $ hg bookmark -d 925d80f479bb
+  $ hg bookmark -d db815d6d32e6
+
 incompatible options
 
   $ hg bookmark -m Y -d Z