I hardly ever use the designer, but when creating custom controls I like to do a cursory check in the design view to make sure nothing barfs too badly.

In the process of creating a menu control, everything worked great in the HTML view and at runtime, but in the designer I was puzzled to see this error:

The error is confusing since 'Menu' is a property of test:MenuControl.  Why would the designer be trying to set a property of the control to a reference to the control? 

Solution:  As it turns out, the problem stems from the fact that the control collection I was using as the 'Menu' property had a setter. That's it. As soon as I took out the setter, the designer was happy.

Stupid designer.