May I raise this question again, now in regard to submission errors.
Every now and then, I get an error when posting an answer or a question, like, the answer is too short, or I forgot to tag the question (I'm happy to get bugged about that, don't get me wrong). I quickly fix the error and submit again, at which point I every time have to face the captcha.
My suggestion: Don't show the captcha for two consequtive submissions that are very close to each other, if the first submission resulted in an error, and the second submission doesn't cause an error. If the second submission is wrong too, then no problem, show the captcha.