Re-writing mariusa's example from https://github.com/caolan/async/issues/114

async-example.js

async.parallel({
  a: function(callback) {
    fs.readFile('a.html', 'utf8', function(err, a) {
      if (err) callback(err);
      
      console.log('read a');
      callback(null, a);
    });
  }, 
  b: function(callback) {
    fs.readFile('b.html', 'utf8', function(err, b) {
      if (err) callback(err);
      
      console.log('read b');
      callback(null, b);
    });
  }
}, function (err, results) {
  console.log('a:' + results.a);
  console.log('b:' + results.b);
});

You know this is incorrect right? The callback would get called twice in the parallel functions. Either place the second callback in an else block or use if (err) **return** callback(err);