Health Monitoring reports “Invalid ViewState”

One problem we have been encountering for nearly a year is a problem which reports ASP.NET errors, but actually does not cause any problems for the user. The error looks like this:

** Events **
---------------
Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 3/23/2010 11:44:44 AM
Event time (UTC): 3/23/2010 11:44:44 AM
Event ID: 6be66d016f1e43b48adbd638e202c698 Event sequence: 3548 Event occurrence: 1 Event detail code: 0

Process information:
    Process ID: 7872
    Process name: aspnet_wp.exe
    Account name: WEBSITE\ASPNET

Exception information:
    Exception type: System.Web.HttpException
    Exception message: Invalid viewstate.

Request information:
    Request URL: http://www.mywebsite.com/WebResource.axd?d=ZUm_ji0BJtTmyRqOTP4MshLi-wjzbbo9jYAp4e20RMiUqPVjOfHps_dhIU5L--PJq9qpm6bObGj7rGo1_bdpliteEndDate_TextBox'),%20null,%20OrderSearch1_bdpliteEndDateParams,%20true)};
    Request path: /WebResource.axd
    User host address: 
    User: 
    Is authenticated: False
    Authentication Type: 
    Thread account name: WEBSITE\ASPNET

Thread information:
    Thread ID: 5
    Thread account name: WEBSITE\ASPNET
    Is impersonating: False
    Stack trace:    at System.Web.UI.Page.DecryptStringWithIV(String s, IVType ivType)
   at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

The URL being requested looks corrupted, it looks like ASP.NET is requested an embedded resource from an assembly, BUT, it is also getting overwritten with some JavaScript which occurs mid page?

It turns out that I wasn't the only one experiencing this problem, but the users also never reported this error either. It turns out that IE8 uses something called a "Lookahead Downloader". It is functionality within the browser rendering that tries to anticipate what resources will be required as the page is downloaded. So rather than downloading the whole page, then scanning it for resources and downloading them individually, it will download them as they are referenced in the X/HTML.

An Microsoft Connect issue has been raised on it, but Eric Lawrence (the author of Fiddler) has posted an article about the problem and how to resolve it.