Failing to call res.send(200) in an express app

ab.sh

$ ab -n 1000 -c 1000 http://localhost:3000/hang
This is ApacheBench, Version 2.3 <$Revision: 1554214 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)
apr_socket_recv: Connection reset by peer (104)

app.coffee

app.get '/hang', (req, res) ->
  console.log 'Request ', n
  n += 1
  # res.send 200

So it's pretty clear that without calling res.send we're just left "hanging".
The console.log usually gets through a few 100s before ab gives up.

My questions:

1. What's happening inside node with those "hanging" connections, are they just sat there eating up memory?
2. What could be causing the apr_socket_recv?
3. The number of connections which occur before the failure seems to vary, is there any significance to it?

Additionally, I found you can pass -r to ab to have it ignore socket errors.
When I use that I actually start to see some requests being completed:

Completed 100 requests
Completed 200 requests
apr_pollset_poll: The timeout specified has expired (70007)

What could be happening here?
Does express or node know to time out and close the connections?