OpenNTPD just ignored the leap second
OpenNTPD has clearly been written by someone who doesn't understand NTP. For example, it advertises incorrect root delay and disperson values, which can cause clients to fail to achieve a majority vote, or to pick the wrong peer to synchronise against. (Earlier versions were even worse, they advertised themselves as being at stratum 0, which could cause synchronisation loops; this has thankfully been fixed, but it doesn't inspire much confidence in the authors' competence.)
I've also found OpenNTP to fail to regulate the local clock on dodgy hardware (it would oscillate wildly, with an amplitude of 3 seconds or so), in situations where the reference ntpd coped just fine.
Folks, do yourself and everyone a favour -- run the reference NTP, run chrony, heck, run some SNTP client, but please avoid OpenNTPD.