Why is AJAX returning HTTP status code 0?

Question

For some reason, while using AJAX (with my dashcode developed application) the browser just stops uploading and returns status codes of 0. Why does this happen?

Answer 1

I found another case where jquery gives you status code 0 -- if for some reason XMLHttpRequest is not defined, you'll get this error.

Obviously this won't normally happen on the web, but a bug in a nightly firefox build caused this to crop up in an add-on I was writing. :)