Validation of viewstate MAC failed

If you got the below error message in your website,

"Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey>
configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster."

Please try to add below code in your web.config file to fix it.

<pages enableViewStateMac="false" />

For  more details, you can see this link.   
http://support.microsoft.com/kb/829743