On a small enough site (even LWN might qualify) the chance of two random sets of client IPs intersecting can be quite low.

Private trackers do this. If they ban a user that geolocates to a certain city and ISP, they'll ban new signups from that city and ISP because there's probably only a few users from the same city and ISP. And then report to their friends at other trackers, that a user with that city and ISP is trying to evade a ban.