This dumb mistake just cost me an hour spelunking around in the debugger:
var status = source.Substring(source.LastIndexOf("/" + 1));
(where source is e.g. "http://foo.com/status/all-is-good")
Fortunately the ramifications were picked up in the acceptance tests, but the root cause wasn't at all obvious from such a high level.
Lesson for the day - code is never too trivial to warrant unit testing.