Here is another issue that a customer had that I thought would be helpful to others.

Problem

The customer said they are getting Exception type: System.IndexOutOfRangeException every minute or so when accessing their web site.  The error looks like:

System.IndexOutOfRangeException: Index was outside the bounds of the array.
  at System.Web.UI.WebControls.GridView.LoadControlState(Object savedState)
  at System.Web.UI.Control.LoadControlStateInternal(Object savedStateObj)
  at System.Web.UI.Page.LoadAllState()
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Resolution

This is trying to load ViewState.  ViewState is not necessarily compatible between different versions of the runtime, including service pack changes. A given application, even if distributed in a web farm, must be running on the same exact runtime version on all servers.  This includes the bit-ness.  So they all need to be x86 or x64 as on the same version (Service Pack, etc).