DOOM Captcha
While this is too simple (even the website admits that a bot could be written in virtually no time to break it), it does make me wonder if you could have a WarioWare CAPCHA.  A minigame with extremely vague description that you have to react to quickly to pass.<p>Thinking about it some more it fails the primary criteria for a CAPCHA: it needs to be harder to write the bot that cracks it than each test case, by at least a couple orders of magnitude.  Otherwise bot makers will just create custom bots for each contingency.  They have more time and funding than you do.