I've experienced this first-hand. I used to always get the checkbox, but then one day I had to download a large series of related files from a website that used reCAPTCHA. I did all the clicking manually, but in a very repetitive and bot-like fashion, by opening a series of 10 new tabs at a time and then performing the same series of clicks on each tab to get to the download link. After a few minutes, I stopped getting checkboxes and started getting increasingly more difficult CAPTCHA challenges.