After calling it a night with a working site, I get up to get to work and see Exception from HRESULT: 0x80131040 when loading the site. The stack trace is below.
C#/VB.NET Code:
[FileLoadException: The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]
System.ServiceModel.Activation.HttpModule.ProcessRequest(Object sender, EventArgs e) +0
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +128
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +76
So I logged onto the server and see a Windows update wants me to reboot. I reboot the server and the site is working again. What a scare!