I've been tweaking my proxy .pac file, and updating the AirPort proxy settings several times. At some point it seemed that OS X began ignoring the proxy settings completely.
System Preferences > AirPort > Advanced > Proxies I see the correct file url and the
Automatic Proxy Configuration is checked (yes, I remembered to "Apply" and save my changes), but browsers are ignoring the proxy. In addition, I get this:
$ networksetup -getwebproxy AirPort Enabled: No Server: Port: 0 Authenticated Proxy Enabled: 0
Even if I try using the command line with:
$ networksetup -setautoproxyurl AirPort http://path/to/my.pac
the command exits with no error message and the proxy still isn't considered nor the
-getwebproxy command show that a proxy is active.