Recently I ran in to a small bug in Internet Explorer where on an empty cache it would take minutes for script.readyState to equal 'complete' after the script had been successfully loaded  from the server. But if you refreshed the page , script.readyState would equal complete on page load.

var script=document.createElement('script');
script.setAttribute('type','text/javascript');
script.setAttribute('src',protocol+'//script.js');

//internet explorer
script.onreadystatechange=function(){
	if (script.readyState == 'complete') {
		alert('ie');
	}
}
//firefox
script.onload=function(){
	alert('ff');
}
document.getElementsByTagName("head")[0].appendChild(script);

 So the solution is to change the script.readyState to test for the loaded state.  It seems that on an empty cache the state equals loaded right after the browser has loaded the script from the server.

//internet explorer
script.onreadystatechange=function(){
	if (/^(complete|loaded)$/.test(script.readyState)) {
		alert('ie');
	}
}

Check out the attachments to see the bug in action.