With everyone raving about YSlow I used it to prune a new project I was doing. Well, after the results worked just great with Safari and Firefox I was thoroughly confused why it wasn't working correctly in IE. The problem was that my Script.aculo.us routines weren't working, and more specifically I was getting really weird errors when the page was loading, before any of the routines executed. Some of the errors were very vague, e.g. "'Class' is undefined", and "Effect.toggle is undefined".