Certain behaviors common to Third Party Tag scripts will not work as asyncronous JavaScript or HTML, or after the DOM has finished loading.
For example, a Doubleclick Floodlight tag might look like this:
<!--
<script type="text/javascript">
var axel = Math.random() + "";
var a = axel * 10000000000000;
document.write('<iframe src="//0.fls.doubleclick.net/activityi;src=0;type=fake_tag;cat=fake_tag;ord=' + a + '?" width="1" height="1" frameborder="0" style="display:none"></iframe>');
</script>
<noscript>
<iframe src="//0.fls.doubleclick.net/activityi;src=0;type=fake_tag;cat=fake_tag;ord=1?" width="1" height="1" frameborder="0" style="display:none"></iframe>
</noscript>
<!-- End of DoubleClick Floodlight Tag: Please do not remove -->
Because this uses "document.write" to create an iframe, this can cause an error- you can't write to a document after it has completed.
You can use document.body.appendChild to add something to the page after it has finished loading.
var axel = Math.random() + "";
var a = axel * 10000000000000;
var dcIMG = document.createElement('iframe');
dcIMG.setAttribute('src', "https://0.fls.doubleclick.net/activityi;src=0;type=fake_tag;cat=fake_tag;ord="+a);
dcIMG.setAttribute('height','1');
dcIMG.setAttribute('width','1');
dcIMG.setAttribute('Border','0');
dcIMG.setAttribute('style','display:none');
document.body.appendChild(dcIMG);