>>I know this can happen if the object being deserialized is not the same class represented by the serialized data (or is the same class but the class definition has changed). Did you post the entire stack trace? Is this happening within a try/catch?
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>I have no idea where this happens. This happens even before it goes in my code. So, this is at the higher level. This is similar to the other two messages I get occasionally. The user does not see any error and everything is ok. But, my framework detects something and logs it.

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....
Just a thought,
Viv