Viewstate MAC failed

When you deploy your ASP.NET website to production you may get an error saying the "Viewstate MAC failed" This happens if your production environment is a web farm.  To fix it you need to add a machineKey entry to the system.web section of your web.config as described here by jnet:

http://www.nopcommerce.com/boards/t/9548/system-logs-what-does-the-error-mean-.aspx