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>