>>Any chance this is related to the ViewState issue? I guess that if you remove the validation then .NET will assume the Viewstate content is OK and attempt to deserialise it. If someone, for malicious purposes or otherwise, has modified the ViewState then you might see this error....
>
>I still have ViewState on.
>
>I assume this is when certain browsers cannot send the entire HTML to the server or that a connection failure would happen along the way.

Not aure. I'd expect that,regardless of the cause of the corruption, if ViewState Validation is enabled then that should catch a problem before the deserializer kicks in. Only an assumption though.....