Even the most complex captchas can be bypassed, if not tested thoroughly enough.
Some time ago, I implemented a captcha on my comments form for news posts, to stop spam bots. My captcha is quite complicated, and sometimes generates images that even humans strugle on, yet for some reason I managed to get 10788 spam comments!
I immediately tested my captcha - tried with no value, a wrong value, a right value, only the right one worked.
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