The Google404Enhancement is used on the page not found error to help find a page.
I don't know anything about fixurl.js where does that come from?
One thing I can mention that I discovered today is that if an error occurs when trying to load a UserControl that is missing it also results in a page not found error which is not intentional. The issue is that we are using part of the error text to detect a page not found error because it doesn't throw a specific exception but a generic HttpException. So what happens is the File doesn't exist text also exists in the error when a UserControl is not found. So for example on my local machine I had moved things around after we moved our source code to Codeplex and I had a page on my local machine with a custom module that was not under source control and did not exist on disk in the new location I was working from. So the page containing the missing Module was returning a page not found error even though the page does exist, because the error message about the missing UserControl is so similar to the error message when a page is really missing. I'm looking into a fix for this. It is a shame that the runtime doesn't have a more specific exception than HttpException when a page is not found, it makes it difficult to trap the error.