Invalid postback or callback argument

Have you ever find this message when editing a page in MOSS or making postback?

"Invalid postback or callback argument. Event validation is enabled
using <pages enableEventValidation="true"/> in configuration or <%@ Page
EnableEventValidation="true" %> in a page. For security purposes, this
feature verifies that arguments to postback or callback events originate
from the server control that originally rendered them.

 I did and I found that when designers made the page layout, they included a form tag inside it.

This form tag originated the problem.