In Silverlight 3 you may get the following warning for your user control:
The property 'AnExistingProperty' does not exist on the type 'YourControl' in the XML namespace 'clr-namespace:your.namespace'. C:\YourApp\MainPage.xaml
Of course the property does exist, and you keep wondering why the computer does not like you again. Why?
The answer is of course that the programmer who wrote the message was lazy. You are not missing a property, but you are missing a Setter or Getter for that property.