Error Throwing in Web Service
I didn't realize that error handling for HTTP requests got to the web service level. If you've deployed an app to a remote server that gets an error, you found that the error handling capabilities provided in the <customErrors> element in the config file catch the error and redirect to the error page (if this element is setup to do so; you can also catch errors in the Page_Error event).
What I found out was that errors in web service calls to the client side also get filtered out on a remote server. The error comes back as a 500 error with a generic message. ASP.NET AJAX is smart enough to handle failures and respond accordingly, but those errors get caught on the server and a generic gets sent to the client.
I found this out because I was displaying the error message and using exception throwing as a validation of sorts, displaying the message in the client. This didn't work however.