ASP.NET Ajax document.getElementById is null or not an object

This has taken me three days to figure out. For what seemed like absolutely no reason, my ajax enabled application just stopped working. I went back through all my changes to see what it could have possibly have been to make the application start throwing the javascript document.getElementById('') is null or not an object error, and specifically, why it was doing this on document.getElementById('HEAD[0]').

I eventually found a secton of html that I had commented out with html comments, and I had included in this an asp.net ajax tag.

Removing these comments fixed the problem!