Cloudflare’s DNS proxy software misbehaved on New Year’s Day, because of the extra second that was added to December 31st: <http://arstechnica.co.uk/security/2017/01/cloudflare-leap-second-software-panic-snafu-new-years-day/>. Because they occur so rarely, it seems very difficult to test software to work correctly with leap seconds. This is one reason why some groups propose getting rid of them altogether.