Validation of viewstate MAC failed

Programming, error messages and sample code
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