navigation to another page in chrome while xhr is active causes "abort" event;
also, ESC key in Chrome while page is loading aborts all xhrs... and "abort" events are fired;
ESC key in FF also stops xhr activity(https://bugzilla.mozilla.org/show_bug.cgi?id=614304), but "error" event is fired for FF.

i know, this cases may be not interesting, but some browser may implement such thing to stop xhr activity....

for "ontimeout", are you sure, that "no-timeout" is a default timeout?