I've just restructured a solution and moved project folders around a bit (outside of Visual Studio). When I loaded the required packages from NuGet, I encountered a problem.
In design view a control from this http://nuget.org/packages/Extended.Wpf.Toolkit package generates an error in the error list (Object reference not set to an instance of an object).
When browsing through the forum for the package several thread mentioned unblocking the dll to remove the error.
When I started looking around, my solution to find the dll there was no unblock button. So maybe this is a problem with NuGet