app.js

var express = require('express'),
    app = express();

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

app.get('/', function(req, res) {
  res.send(500);
});

module.exports = app;

package.json

{
  "name": "supertest-issue",
  "version": "0.0.1",
  "private": true,
  "dependencies": {
    "express": "3.x",
    "expect": "^0.1.1",
    "expect.js": "^0.3.1"
  },
  "devDependencies": {
    "mocha": "*",
    "supertest": "*"
  },
  "scripts": {
    "test": "node_modules/mocha/bin/mocha ./test.js"
  }
}

test.js

var request = require('supertest'),
    expect = require('expect.js'),
    app = require('./app');

describe('check status 500', function() {

  it('is correct', function(done) {
    request(app)
      .get('/')
      .expect(500, done);
  });

  it('is not correct', function(done) {
    request(app)
      .get('/')
      .expect(404)
      .end(function(err) {
        expect(err).not.to.be(undefined);
        done(err);
      });
  });

});