We ran into a rather silly issue with a recent Silverlight deployment. In our RIA services is it not uncommon for use to throw custom exceptions from the RIA side and then have our Silverlight application display them to the user.
For instance we would throw a custom exception if there was an issue with the login and we would expect the following to be "User and password is invalid" to be displayed. However we were getting "Load operation failed for query 'Login'. Exception of type 'System.ServiceModelServices.Client.DomainOperationException' was thrown."
What we noticed is anytime we through a custom exception it was being returned as DomainOperationException instead of the custom exception type. This drove us nuts because it was working in each of our development environments but not on this integration machine we were using for testing. We did some searches on DomainOperationException but really could not find much that helped. We did find this blog (http://weblogs.asp.net/fredriknormen/archive/2009/12/08/wcf-ria-services-exception-handling.aspx) however it was poorly written.
Well the solution was really simple. All we had to do was make the following modification to our web.config where the RIA Services were hosted.