This error occured on my application hosted with godaddy.com

 Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that  configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that  configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.



Solution
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This can happen if a postback occurs before the enitre page has been sent to the browser.

The reason is that ASP.NET create a MAC of the viewstate in the page (to make sure that none tampers with the viewstate) and that MAC as sent last in the page. If the page is posted back before the MAc has reached the browser ( = without the MAC), ASP.NET cannot validate the viewstate and throws an exception.

There are (at least) two solutions to this problem:

   1.  Make the page load faster. I tried it and it was very slow.
   2. Turn of viewstate validation either in your page (EnableViewStateMac="false") or in Your web.config :  .