When I debugged an ajax web service call to SharePoint listdata.svc, I often get a HTTP 400 error, which made me think something wrong with REST content format. It turns out it is because Fiddler Request Builder will not take white space!
Instead of typing in %20, a easy way is, first use IE browser and then in Fiddle Sessions window, copy its url to its Request Builder.
Secondly, if your web site makes another http call, the second http traffic will not be tracked if you run your web site from Fiddler. But they are tracked if run from browser.
Last, but not least, Fiddler 2 seems to work fine now with http://localhost/ , but is kind of tricks: if your website make a http call such as
request.set_url(http://localhost/_vti_bin/ListData.svc)
the traffic of this request will not be tracked in Fiddler
