Craft Commerce - No closure name "Customer"
In the latest beta update a change was made to how addresses are managed. You can no longer access the customer from the address model.

So the following does not work:
{{address.customer}}

There was only one location we did this in the example templates and that was in the file: craft/templates/commerce/_includes/address.html. We removed it from the example templates, so I expect that possibly you have not updated your example templates from the latest release. You will need to manually download the latest release from http://craftcommerce.com/download to get the latest example templates, as the 'one click update' magic does not update the example templates automatically.

If you need to access the customer for the order, it is always best to do it from the order and not the order's addresses – like this:

{{order.customer}}