CORS request isn't allowed despite headers being set - security

Question

I get the following error trying to do an XHR request. I've setup CORS response headers, but apparently something is wrong. Can anyone spot the error?
XMLHttpRequest cannot load http://domain.dev/path. Origin http://mobile.dev is not allowed by Access-Control-Allow-Origin.
Preflight Request Headers (OPTIONS)
This is a pre-flight request, to verify the request.
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Access-Control-Request-Headers:origin, x-pre-process, x-requested-with, x-client, x-client-version, accept, x-session
Access-Control-Request-Method:GET
Cache-Control:max-age=0
Connection:keep-alive
Host:my-app.dev
Origin:http://mobile.dev
Pragma:no-cache
Referer:http://mobile.dev/
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.2 (KHTML, like Gecko) Chrome/22.0.1211.0 Safari/537.2
Response Headers
This is the server response to the pre-flight request.
Access-Control-Allow-Headers:origin, x-pre-process, x-requested-with, x-client, x-client-version, accept, x-session
Access-Control-Allow-Methods:GET, POST, PUT, PATCH, DELETE
Access-Control-Allow-Origin:http://mobile.dev
Access-Control-Max-Age:3600
Cache-Control:max-age=0, private, must-revalidate
Connection:close
Content-Type:text/html; charset=utf-8
ETag:"7215ee9c7d9dc229d2921a40e899ec5f"
X-Request-Id:3fca5f24077bcbd1351d552edf311f82
X-Runtime:0.014551
Real Request Header
This is the real request, which is canceled by the browser due to the error quoted above.
Accept:application/json, text/javascript, */*; q=0.01
Cache-Control:no-cache
Origin:http://mobile.dev
Pragma:no-cache
Referer:http://mobile.dev/
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.2 (KHTML, like Gecko) Chrome/22.0.1211.0 Safari/537.2
X-Client:mobile
X-Client-Version:1.6
X-Pre-Process:underscore
X-Requested-With:XMLHttpRequest
X-Session:j9y01yw33txmdbcz1ao258uy7bzjlm

Answer 1

It seems that proper handling of the pre-flight OPTIONS request is necessary, but NOT SUFFICIENT for cross-site resource requests to work.
After the OPTIONS request comes back with satisfactory headers, all responses to any subsequent requests to the same URL also have to have the necessary "Access-Control-Allow-Origin" header, otherwise the browser will swallow them, and they won't even show up in the debugger window.
So it will look like the browser cancelled the request because of some problem in the OPTIONS response, but actually, the browser is looking at the response headers from the real request and then rejecting them.
(Answer copied from my own similar question Access-Control-Allow-Origin header not working - What am I doing wrong? in case it's the same thing)