If you are testing your code, or going to other pages with Silverlight running and seeing this error:
It is not your fault!
This is the result of a recently discovered bug that we know how to fix. We are working hard to get that fix out so that you won't run into this problem.
For now, the work around is this: if you try the page again, you'll usually find that it will load just fine (or the second or third time .)
I will post as soon as we have more information, and I apologize for the inconvenience this may be causing.