This one may sound weird if you haven't been to editathons with newbies. "You only have to solve the CAPTCHA once", you think. Well, yes. If you do it correctly on the first try. In the several editathons I've been where I've instructed people on how to create a new account, they almost never manage to do it on the first try -- either because they try a username that is already in use on another wiki (see related T90732 and which I noticed we //don't// have AJAX warnings for) or because their passwords don't match up. What happens then is that the form is loaded again, with an error message on top, while the password fields are cleared and they get a new CAPTCHA to solve. I strongly believe that if they already soved a CAPTCHA -- even though the pasword or username were wrong -- the CAPTCHA has served its purpose in proving that the user is in fact human, so they should only have to fill it in once.