This is simple way to reproduce the socket hang up. Hope it can help.

socketHangUp.js

// npm install express supertest
// node socketHangUp.js

/**
 * Express server.
 */
var express = require('express')
  , request = require('supertest')
	;

var app = express();
app.configure(function() {
  app.use(express.bodyParser());
  app.use(app.router);
});

app.get('/', function(req, res) {
  res.send('hello world');
});

/**
 * Requests.
 */
var i 	  = 0
	, max 	= 1000
	, reqfn = function () {
			request(app)
			  .get('/')
			  .end(function(error, res){
			  	console.log('i = ', i++);
			  	if (i<max) reqfn();
			  });

			request(app)
			  .get('/')
			  .end(function(error, res){
			  	console.log('i = ', i++);
			  	if (i<max) reqfn();
			  });
	};
reqfn();

/*
i =  1996

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: socket hang up
    at createHangUpError (http.js:1417:15)
    at Socket.socketOnEnd [as onend] (http.js:1513:23)
    at Socket.g (events.js:175:14)
    at Socket.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:870:14
    at process._tickCallback (node.js:415:13)
*/

• What version of express does it use?
• What is supertest (in require('supertest');)

I ran:

$> npm install express supertest
$> node sockethangup.js 

And this is what I got:

i =  508

events.js:66
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: listen EADDRINUSE
    at errnoException (net.js:768:11)
    at Server._listen2 (net.js:908:14)
    at listen (net.js:935:10)
    at Server.listen (net.js:984:5)
    at Test.serverAddress (/private/tmp/node_modules/supertest/lib/test.js:65:18)
    at new Test (/private/tmp/node_modules/supertest/lib/test.js:44:12)
    at Object.module.exports.methods.forEach.obj.(anonymous function) [as get] (/private/tmp/node_modules/supertest/index.js:29:14)
    at reqfn (/private/tmp/sockethangup.js:28:10)
    at reqfn (/private/tmp/sockethangup.js:38:22)
    at Test.assert (/private/tmp/node_modules/supertest/lib/test.js:195:3)
    at Test.end (/private/tmp/node_modules/supertest/lib/test.js:124:10)

I'm on node 0.8.8 and express is v3.2.0

Ok my bad, I had another node instance running elsewhere. Now I have this different error (still not a socket hangup:)

i =  1016
i =  1017

events.js:66
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: connect EMFILE
    at errnoException (net.js:768:11)
    at connect (net.js:644:19)
    at Socket.connect (net.js:709:9)
    at dns.js:71:18
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)