We were actually just discussing the "what if I trip their filter" concern at our morning meeting. Full disclosure: my company (as the username implies) builds FunCaptcha, a CAPTCHA alternative. Your concern, to us, is a very valid one and has been a driving force behind our own design and mentality. Our lead designer is (understandably) passionate about this so he actually wrote a few words on the blog that dives pretty deeply into the topic, if you're inclined: https://www.funcaptcha.co/2014/12/04/killing-the-captcha-wit....