title: CFNetwork internal error
body: <p>I'm creating my own NSURLProtocol with a caching system and I'm seeing this on the console:</p>
  
  <p>4676: CFNetwork internal error (0xc01a:/SourceCache/CFNetwork_Sim/CFNetwork-609.1.4/Connection/URLConnectionClient.cpp:558</p>
  
  <p>Any idea what is causing this?</p>
  
  <p>There's no indication that any of the traffic is not being handled correctly at the current point where this shows up. The response is cached, the server sends back 304 not modified and I load the cached response and then while stepping out another thread logs that error.</p>