Only somewhat related, there was a "voting" website once that let people cast a vote for something, up to one vote per minute. I was corralled into the voting by family, and after 30-60 minutes decided that this was really dumb, so I wrote a tiny webpage with a form with the key ids for what we were voting for, and set the page to refresh every minute, thereby casting my votes automatically.
After some click-fraud (related to google ad words, not my thing) the admin added a captcha to every 5th vote that one cast. After studying the javascript for a bit I determined that the captcha was handled by a third party where the javascript would query if the captcha text was correct and the third party would respond yes or no. I made a new program in C# that would do an http post to submit the vote on a timer. If a captcha was required, my program would very helpfully inform the website that yes, I had indeed typed it correctly (I promise!).
And thus the captcha was bypassed.
Aftermath, for the curious:
Somewhat amusingly, the web server powering this website was underpowered and would stop serving pages (http GET) for up to a few hours at a time. However, it would always process my program's incoming POSTs. So the website would go down and when it came back up our faction's votes would have jumped several hundred while the other faction's stayed the same. This caused all manner of chaos in the (very crazy, very rabid) user base. It remained entertaining for a few rounds of voting (each round lasted a week), and then I got bored with it and washed my hands of the whole mess.
The site creator was a med student who's course load was picking up, and he basically dumped the site a month or two after the captcha update.
Experimenting after everyone but about (literally) two users had abandoned the site, I determined that there was no validation for which voting period would accept votes. It was possible to log votes against a future period, and when that period finally arrived the voted for item would have a head start of however many votes had been cast.