Here is another issue that a customer had that I thought would be helpful to others.
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.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
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).