ERROR: The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again

I was getting above error while doing altertask on SPWorkflowTask.AlterTask method.

Solution:  we need to write altertask code under SPSecurity delegate and need to write code for Allowunsafeaccess.

http://sansanwal.blogspot.com/2009/08/security-validation-for-this-page-is.html

http://support.microsoft.com/default.aspx?scid=kb;EN-US;970192