If you are using Silverlight 4, chances are you have encountered the nasty exception while parsing a XAML file, something on the lines of : “Parser internal error: Object writer ‘xClassCanOnlyBeUsedOnLoadComponent’”. It can be quite frustrating to debug this because you can’t step into the Load method and the line number details that the exception provides is basically useless.
Whenever you see the term xClassCanOnlyBeUsedOnLoadComponent, look for x:Class in your XAML file. Check whether the namespace specified is correct and your code behind (xaml.cs) also uses the same namespace. If you are using a XAML file only without a code behind file (xaml.cs), then you do not need the x:Class tag in your XAML file. Remove this and the exception should go away.
Let me know if this solves the problem. That being said, Microsoft should really work on improving its exceptions. The least they could do is provide accurate information.