Items collection must be empty before using ItemsSource

In silverlight or WPF if you get the error:

"Items collection must be empty before using ItemsSource"

make sure you aren't populating the control with both the ItemsSource and adding children to the control in XAML.  In my case, I was setting ItemsSource after a call to a WCF service.  I had mistakingly also added a placeholder object in my XAML until I had written the WCF call.  Oops. 

Hope this helps somebody,

Scott