Is there is any conditions that could lead to ontimeout triggered when timeout is set to 0?

var xhr = new XMLHttpRequest();
xhr.open('POST', '/something', true);

xhr.timeout = 0; // No timeout

xhr.ontimeout = function (e) {
    // Any circumstances to get here?
};

xhr.send(...);

Some sort of slow connection? Or very long internal browser connect/read timeout?

I found some users of the site I maintain fall into this problem and want to figure out why.