Internet explorer seems great at providing errors that don't show up in any other browser. This time it seems that I had found one that lies deep within the "magic" of ASP.NET AJAX. The error was

ASP.NET Ajax client-side framework failed to load
Sys is undefined
   

I tried first to see if there were any other weird javascript errors that might be causing the error in the first place, but after resolving everything else, I realized that my next best bet would be to Google, as I usually do to resolve the error. That seemed to lead me down a wrong path, as ASP.NET articles seem to say that .NET AJAX libraries might need to be installed. However, we were seeing the error in .NET 3.5 sites running on IIS 7 servers, and .NET AJAX comes installed automajically in ASP.NET 3.5 binaries. So that wasn't it.

Turns out that I was requesting the site on the wrong domain, and ajax doesn't like that. We set our cookies on www.dev.somesite.com, and I was testing the site at dev.somesite.com. Seems that if you are viewing a page on a subdomain that is different than the one the cookies are set on, you will see these kinds of errors. So, the lesson learned is that, AJAX must be requested on the same domain that the cookies are set on, or you may see these errors even with AJAX installed.