I have made a pull request with a test that fails.
#509

RangeError Maximum call stack size exceeded

It is basicaly this

var q = async.queue(function(task,cb) { cb(); })
q.pause();

for (var i=0; i< 5000; i++) {
  q.push({ index: i });
}

q.resume();

When cb is called there can be RangeError's
I don't know enough to figure out why though.