>>But you are right, the client app needs to be expecting it, otherwise the user would just go Huh?
>
>And it would be pretty unfair to assume the client can and wants to parse XML. Like I said, trap errors you expect, and return failures in XML, but if an unexpected error occurs (bug, syntax error, ect.) go with the standard stuff.

Well, you can safely assume nowadays that any client can parse XML, it is up to the UI programmer to put the logic for parsing before showing the end user any error message.

Though I see your point that an exception of that kind wouldn't necessarily require special processing.