I just spent a few minutes trying to figure out an odd error I was having in trying to add Kevin's WPF Bag-o-Tricks to my MemoryGame project. Apparently if you make use of such a 3rd party control (or anything with a different app domain) in one XAML file and then attempt to put that XAML page into another XAML page (whose project does not contain a reference to said 3rd party library) you will get this lovely error: