diff -r 548478efc46c -r bce5ebe72859 mercurial/commands.py
--- a/mercurial/commands.py	Thu May 04 11:51:07 2017 +0900
+++ b/mercurial/commands.py	Mon May 22 19:18:12 2017 -0400
@@ -968,6 +968,11 @@
             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"))