This method internally calls the Uri.IsWellFormedUriString method. When you call this method on a relative URL containing a fragment it returns false. There is a bug on MS Connect which is closed with the by design reason.
When you use this method on an absolute URL (with a scheme like http/https) the method behaves as expected. I think the reason is that the Uri class is intended to work not only with HTTP URLs. When you don't specify the protocol (relative URL), a generic URL parser is used which doesn't allow fragments.
So I guess you have two possibilities:
http://foo.com/t/test-team-3/tasks#/lists/15) because anyway if you are calling this method on a relative URL we can expect that it is a local URL.