As an ordinary web user, if CloudFlare suspect you may be malicious (your IP address being on a spam blacklist seems to be enough) they will show you a warning

As an ordinary web user, if CloudFlare suspect you may be malicious (your IP address being on a spam blacklist seems to be enough) they will show you a warning that you should scan your PC for viruses and ask you to complete a captcha. (They linked to a report on a spam blacklist claiming my IP address had been used to send spam during the past week. I'd had my IP address for about twelve hours. Thanks, TalkTalk!) As a web site operator, you have the option of switching this behaviour off.