Are you running Silverlight and using a Popup and a “drop-down” control like a ComboBox (or DatePicker, TimePicker, etc) as a child? You may be seeing some odd behavior when you expand the ComboBox. The “drop-down bit” ends up positioned from the left of your screen, not where it is supposed to be positioned. Hmmm.
Turns out the solution is easy, but not easy to find. I’m hoping this post will help someone else out and prevent others from beating their heads against a wall. Ready?
Turn off the DropShadowEffect on the Popup.