I thought this item would be related to that very popular item... I saw it mentioned in discussion.
At work I just recently managed to trip it three times in one day, which I consider a record since in the past I've encountered it at most a few times a week. What's more infuriating is that my queries were far less complex than the ones that tripped it before (trying to find information about some API constants), basically one quoted term and one site: modifier. I can understand if I was querying 24/7 and hogging their servers (in which case a nice "please slow down" message would be much better), but it tripped within a few minutes of, admittedly intense, Googling.
The security device may notify the attacker device that the solution is incorrect regardless of whether the solution is actually correct.
In Google's case, it doesn't do that at all --- solve one CAPTCHA successfully and all you get is another, immediately. If you actually do deliberately give the wrong answers, it does tell you they were incorrect.
I wonder what can be done to stop it from doing that, besides the old tactic of evading IP bans by changing IP --- it's somewhat creepy to think that it's probably capable of detecting that too and banning the entire subnet. On second thought, it might be worth it... if it means I can get thousands of others blocked from Google for a nontrivial amount of time, all the more mouths to complain and maybe force some reconsideration. I am not a robot. I am not a competitor scraping your pages or doing anything else against your ToS. I am just an intelligent human with over two decades of Internet searching experience enthusiastically using your service for the exact purpose it claims to do: to find something on the Internet.