When working with multiple OpenERP instances in different stages, you can be sure your customer will one day mix up your pre-production and production instance, which can have catastrophic effects.

A quick and dirty hack to prevent such events is to add a hard-coded warning message to all content produced by OpenERP:

The result above was produced on OpenERP 6.0 thanks to the following patch on the header.mako template file: 

--- addons/openerp/controllers/templates/header.mako.orig       2012-02-20 11:13:08.228864937 +0000
+++ addons/openerp/controllers/templates/header.mako    2012-02-20 11:12:41.361480113 +0000
@@ -115,3 +115,18 @@
         });
     });
 </script>
+
+<!-- Custom header banner -->
+</tr><tr>
+<style type="text/css">
+    #warning-banner {
+        background-color: #c00;
+        color: #fff;
+        text-align: center;
+        font-weight: bold;
+        padding: 0.6em;
+    }
+</style>
+<td id="warning-banner" colspan="3">
+    <p>Warning: this is a pre-production OpenERP instance.</p>
+</td>