concrete5/concrete5 — pull request #558 by Remo (2012-06-22 11:19:27+00:00)
Title: 404 status not returned

It's a change Mnkras suggested but I couldn't find a pull request for it, details here:
http://www.concrete5.org/developers/bugs/5-5-1/404-error-pages-does-not-return-the-correct-status-code/