I was crawling http://www.imenik.hr (more specific: http://www.imenik.hr/imenik/trazi/1/ptt:51551.html) and it was working. I used PHP Symfony crawler, I even used proxies. I was testing it locally so I tried without proxies.
I added 5 seconds of sleep time between each requests, I tried random sleep between requests, I tried 10 seconds. After only few requests it gave me this page http://www.imenik.hr/imenik/captcha.html
I tried incognito and it gave me the same page. I guess it ties my crawling with IP address. How does it even recognizes those requests as non human?