Is there an easy way to simulate an invalid ViewState?
Of course: just add a simple javascript function to the page to change the hidden __VIEWSTATE input.
You can also use fiddler
It can simulate postback very easily and you can modify this post.
Here's a jQuery way to implement Joel's suggestion:
<script language="javascript"> $(document).ready(function() { jQuery('input[@name=__VIEWSTATE]').val("this is now bad"); });