Invalid Viewstate

Answer 1

This is a bug caused by IE8 users, you can read about it here: http://blogs.msdn.com/ieinternals/archive/2009/07/27/Bugs-in-the-IE8-Lookahead-Downloader.aspx

The basics are that when IE8 processes the page, it sometimes just leaves 4kb out of it, and if your <script src="ScriptResource.axd?.... started and gets truncated, when the page is rejoined with 4k missing, up until the matching quote to close the src="" is used, resulting in a very strange request to your server.

Update: It's been fixed now with KB980182 (4/1/2010)