Few days ago, Gunnar told me about a quite curious captcha:
But, in the other hand, it had been implemented insecurely. With just one answer, you can submit many times:
Furthermore, I notice that the captcha was precomputed and, therefore, finite and reused. I made more than 15,000 requests and I had less than 5% unique (there is no motivation to solve 700 differential equations :P).