Index: mozilla/webtools/bugzilla/editusers.cgi
===================================================================
RCS file: /cvsroot/mozilla/webtools/bugzilla/editusers.cgi,v
retrieving revision 1.142
diff -u -r1.142 mozilla/webtools/bugzilla/editusers.cgi
--- mozilla/webtools/bugzilla/editusers.cgi
+++ mozilla/webtools/bugzilla/editusers.cgi
@@ -683,7 +683,7 @@
 } elsif ($action eq 'activity') {
     my $otherUser = check_user($otherUserID, $otherUserLogin);
 
-    $vars->{'profile_changes'} = $dbh->selectall_arrayref(
+    my $list = $dbh->selectall_arrayref(
         "SELECT profiles.login_name AS who, " .
                 $dbh->sql_date_format('profiles_activity.profiles_when') . " AS activity_when,
                 fielddefs.description AS what,
@@ -697,8 +697,19 @@
         {'Slice' => {}},
         $otherUser->id);
 
+    $vars->{'profile_changes'} = $list;
     $vars->{'otheruser'} = $otherUser;
 
+    my $incomplete_data = 0;
+    foreach my $entry (@$list) {
+        my ($who, undef, $what, $removed, $added) = @$entry;
+        next unless $what eq 'Group';
+        $incomplete_data = 1, last if $removed =~ /\?$/;
+        $incomplete_data = 1, last if $added =~ /\?$/;
+    }
+
+    $vars->{'incomplete_data'} = $incomplete_data;
+
     $template->process("account/profile-activity.html.tmpl", $vars)
         || ThrowTemplateError($template->error());
 
Index: mozilla/webtools/bugzilla/template/en/default/account/profile-activity.html.tmpl
===================================================================
RCS file: /cvsroot/mozilla/webtools/bugzilla/template/en/default/account/profile-activity.html.tmpl,v
retrieving revision 1.2
diff -u -r1.2 mozilla/webtools/bugzilla/template/en/default/account/profile-activity.html.tmpl
--- mozilla/webtools/bugzilla/template/en/default/account/profile-activity.html.tmpl
+++ mozilla/webtools/bugzilla/template/en/default/account/profile-activity.html.tmpl
@@ -33,6 +33,8 @@
   #   what: string; the description of the field which was changed
   #   removed: string; the removed value (maybe empty string)
   #   added: string; the added value (maybe empty string)
+  #   incomplete_data: boolean. True if some of the data is incomplete
+  #                    (because it was affected by an old Bugzilla bug.)
   #%]
 
 [% title = BLOCK %]
@@ -44,6 +46,16 @@
   title = title
 %]
 
+[% IF incomplete_data %]
+  <p>
+    There used to be an issue in <a href="http://www.bugzilla.org/">Bugzilla</a>
+    which caused accurate activity data not to be kept for Group membership
+    changes.  That has been fixed, but some data was already lost in
+    your activity table that could not be regenerated.  The changes that the
+    script could not reliably determine are terminated with '?'.
+  </p>
+[% END %]
+
 [% PROCESS admin/users/listselectvars.html.tmpl
   listselectionvalues = listselectionvalues
 %]