Starting a few months ago we started receiving System.Web.HttpException: Invalid viewstate. errors thru our websites. It started out just happening very rarely and we started getting more and more of the errors. The callstack was usually:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Web.HttpException: Invalid viewstate.at System.Web.UI.Page.DecryptString(String s)--- End of inner exception stack trace ---at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)at System.Web.Handlers.ScriptResourceHandler.DecryptString(String s)at System.Web.Handlers.ScriptResourceHandler.DecryptParameter(NameValueCollection queryString)at System.Web.Handlers.ScriptResourceHandler.ProcessRequest(HttpContext context)at System.Web.Handlers.ScriptResourceHandler.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)   

I looked at similarities of the error and noticed that the user agent was always:

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; InfoPath.1; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)

Also, the query string would contain HTML text in the querystring, like:

d=Q0M5xAMIxE-XI2-0ILfpxYhpmFOEVhqxB_ONfw1nSIFAoH8HCyt--97Kq_DD1JoDIf7tnu8rxs0nJzeGOz1YULX%20%20%20%20%20%20%20%20%20%20%20<table%20style=

So it seemed that IE 8 was causing the error, after research I found that I was right. IE8 was causing the problem. I found this bug report on the issue.

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=434997