The security validation for this page is invalid. - SharePoint 2010

In my case I got this error while activating a feature. You might encounter this error during Feature activation using power shell script or STSADM. The exact error message would be follows

The security validation for this page is invalid. Click Back in your Web browser

, refresh the page, and try your operation again.        

Solution : You need to make sure you have set the AllowUnsafeUpdates to false somewhere in the feature activation code and your code expect it to be true to perform certain actions. You may change it back to False at the end of the feature receiver code.