I still do not investigate problems carefully enough. Is it lack of exposure, or lack of effort? The boss explained it in a few sentences and sorted it out. Looking at those confusing AJAX requests in Google’s browser made me sigh. I really did not know how to read them before; Firefox’s Firebug had been easier for me. At last I learned to use Chrome. The first issue was that writing return false in an AJAX callback did not work in this situation, so it kept failing. The form also used onsubmit="a wrapped function" to validate the username, email, CAPTCHA, and so on. That created the problem: two forms were being submitted at the same time. How could it work? Seeing the request URL made me feel foolish. Why had I not thought of it then?
Life in the Past