Greasemonkey

Hey /prog/
I need some help with greasemonkey scripting.
this is what I got:

var ad = document.getElementById('ad_left');
ad.parentNode.removeChild(ad);

It should work, but doesn't. What's wrong?

var ad;
while((ad = document.getElementById('ad_left')) != null) {}
ad.parentNode.removeChild(ad);

How badly could this fail?