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Once upon a time I hosted a very small phpBB with a couple hundred members and a few posts a day. We had domain specific questions for our captcha, and they cycled between 3-4 different questions. Within a month, the bots had been programmed to answer all the questions on our registration. Being obscure and domain specific did not help.

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How about domain specific pictures? "What is in the picture below?" The put a picture of a CPU heat-sink using the previous example And have many pictures to cycle through. Maybe change them up every couple months. Or number the pictures/questions and have the script use a different batch every week. You just need 52 batches and use ImageMagick to change the checksum and quality randomly. Then change them out once a year. I mean, they can update the bot every week, but there must be some really amazing reason to go through that effort if your site is attracting such people. I could see that happening to a gambling site, but probably not the average site.

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