I use firefox for the vast majority of my web browsing, with IE and Chrome occasionally thrown in. While using IE 7 the other day, I opened up this very site, and to my dismay, discovered a javascript error: Line 89, Object doesn't support this action.

I was surprised, since I tested the site on IE 7 not too long ago and am not using any plugins or 3rd party code (i.e. this isn't a wordpress site). At first, it appeared the Core MooTools was the culprit, as removing that file stopped the error. But that didn't explain why there was no error when I tested it on my development box, which had the MooTools JavaScript included. After a little more head scratching I realized the only difference between the site on my dev box and production box was the fact that I have Google Adsense ads on the live site, but not on the dev site.

Sure enough, removing the ads caused the error to go away. Trying to be a good web developer, I was loading my MooTools scripts at the bottom of the page to prevent locking the browser while rendering the page. When I moved the Core MooTools script into the head tag at the top of the page, the error went away. Not sure the exact details of the problem, but loading MooTools after Google Adsense was causing errors in IE 7...