Today's browser oddity is that Chrome instantly fails a script tag download if the requested URL has already failed a couple of times. It stops issuing network requests and fails the download before even trying. Probably other WebKit-based browsers do the same; I didn't check.
I can see why this behavior would make sense if you think of script tags as part of the static content of a page. If there are ten script tags requesting the same resource, you don't want to issue ten failing requests. However, it caught me by surprise, because I'm trying to use script tags as a way of downloading content over the web.
Browsers sure are a messy platform.