Earlier i faced the following issue – i was creating a Silverlight application using Visual Studio 2010 and Silverlight.
I noticed that after copying and pasting a Style Setter, then editing that Setter’s Property and Value, the XAML Editor displays an invalid error message. Also the error message is not be cleared and refreshed by the XAML Editor.
This problem was corrected and fix was available part of Silverlight 4 tools release. You can download it from Microsoft Silverlight 4 Tools for Visual Studio 2010 and also mentioned in the knowledgebase article (http://support.microsoft.com/kb/2282880).
Microsoft recommends that all Silverlight and WPF developers download and install the Silverlight 4 tools. Additionally the above error condition can be cleared by rebuilding the project if the latest Silverlight 4 tools has not yet been installed!!