There's something to be said for this. It would not be at all difficult to implement. Note that I could not actually throw an exception, since I can't detect the use of Context.set(name, value), but I could log a WARN.
Add a feature request to JIRA. I think its a great idea.
I just got to thinking that it would be neat if ComponentScanner and NamespaceScanner were themselves components that could be overridden. That way it'd be simple for anyone to add their own app-specific validation logic. Does make for a bit of a bootstrap problem, I'll admit.