I had a client report of the an error when previewing, it seemed to occur constantly for a couple of hours when they were working, but has not recurred since.

For completeness I'll report it here, and on the codeplex issue tracker.

If anyone else has this exception hopefully they'll find this / share :-)

I dug in the Windows Event Logs and found the following.

Thread information: 
    Thread ID: 32 
    Thread account name: NT AUTHORITY\NETWORK SERVICE 
    Is impersonating: False 
    Stack trace:    at ClientDependency.Core.FileRegistration.Providers.BaseFileRegistrationProvider.UpdateFilePaths(IEnumerable`1 dependencies, HashSet`1 folderPaths, HttpContextBase http)
   at ClientDependency.Core.FileRegistration.Providers.WebFormsFileRegistrationProvider.RegisterDependencies(Control dependantControl, List`1 allDependencies, HashSet`1 paths, HttpContextBase http)
   at ClientDependency.Core.Controls.ClientDependencyLoader.b__1(ProviderDependencyList x)
   at System.Collections.Generic.List`1.ForEach(Action`1 action)
   at ClientDependency.Core.Controls.ClientDependencyLoader.RenderDependencies()
   at ClientDependency.Core.Controls.ClientDependencyLoader.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)