When does CodeIgniter return 404 headers, exactly?

[eluser]besson3c[/eluser]
[quote author="JoostV" date="1257775922"]A nicely rendered page does not always mean the status is 200, of course.[/quote]

Good point!

According to the usage log exerpt you posted the 404 is returned on URL '/', so the webroot for the domain for this log.

That means the status code for webroot '/' for that domain in your Firebug in the Net panel should also say 404. Your ajax calls also show up in Firebug, so if any of them returned 404 you should see that in your Net panel as well.

Are you sure you are looking at the right Apache user log I have never encountered a situation where the Apache tells me 404 and Firebug tells me 200.

I'm seeing the 404s in both Firebug and my Apache logs, yet the content renders fine as long as I use Prototype's onComplete callback (and I see the correct content within Firebug).

Is there *anything* in CodeIgniter that could generate a false 404 header?