Interesting issue today.  I was attempting to install a SharePoint app today and ran into the following error:

8fff7a1f-f0ea-4d73-8c7d-aa97279b6d20 Stack trace:  
at Microsoft.SharePoint.SPUser.get_UserToken()   
at Microsoft.SharePoint.Lifecycle.SPAppLifecycleEventCalloutTask.CallRemoteEventReceiver(Uri url, SPAppInstance instance)   
at Microsoft.SharePoint.Lifecycle.SPAppLifecycleEventCalloutTask.CallRemoteEventReceiver(SPAppInstance instance)   
at Microsoft.SharePoint.Lifecycle.SPAppLifecycleEventCalloutTask.Deploy()   
at Microsoft.SharePoint.Administration.SPAppTask.DeployOperation()   
at Microsoft.SharePoint.Lifecycle.MonitoredTaskExecution.DoTask()   
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)   
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)   
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)   
at System.Threading.ThreadHelper.ThreadStart()

I Googled the the first line in the error and came across my own blog entry, but it was not helpful in this case because the high trust cert was correct.  This time around, my issue was that the Client ID specified when packaging the app was incorrect.  We have multiple applications and the packaging profiles got mismatched.  In general, it looks like if you have a get_UserToken() issue in the call stack, then you’ve probably misconfigured something.