Formidable fails if the request is already emited. formidable.parse fails if an asynchronous event happens before.

formidable_test.js

var formidable = require('formidable'),
    http = require('http'),
    sys = require('sys');

server = http.createServer(function(req, res) {
  req.pause();
  req.on('data', function(buffer) {
    console.log('data parsed');
  });

  setTimeout(function () {
    sys.puts('fffuuuuuuu!');
    if (req.url == '/upload' && req.method.toLowerCase() == 'post') {
      req.resume();
      // parse a file upload
      var form = new formidable.IncomingForm();
      form.parse(req, function(err, fields, files) {
        res.writeHead(200, {'content-type': 'text/plain'});
        res.write('received upload:\n\n');
        res.end(sys.inspect({fields: fields, files: files}));
      });
      return;
    }

    // show a file upload form
    res.writeHead(200, {'content-type': 'text/html'});
    res.end(
      '<form action="/upload" enctype="multipart/form-data" method="post">'+
      '<input type="text" name="title"><br>'+
      '<input type="file" name="upload" multiple="multiple"><br>'+
      '<input type="submit" value="Upload">'+
      '</form>'
    );
  }, 1000);
});

server.listen(3000);

sys.puts('listening on http://localhost:3000/');