Setting EnableViewStateMac=true is a security measure that allows ASP.NET to ensure that the
viewstate for a page has not been tampered with. If on Postback, the ASP.NET framework detects that
there has been a change in the value of viewstate that was sent to the browser, it raises an error -
Validation of viewstate MAC failed.
Use <%@ Page EnableViewStateMac="true"%> to set it to true (the default value, if this attribute is
not specified is also true) in an aspx page.