I remember at one point in my teens, someone had made a web app that would snag the captcha and show you only the captcha, and you would just endlessly solve captchas, while the application tried different passwords on a backend, and logging any successful logins.

Some of the first bitcoin faucets in 2011, 2012 were bots doing that

Users thought the captcha was antispam prevention for them to receive bitcoin

It was really just the bot forwarding a captcha to continue its spam once solved, posting the user in bitcoin