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.