A couple of days ago this appeared in my system logs

Mar 31 23:59:59  kernel: Clock: inserting leap second 23:59:60 UTC

my first reaction of course was "great! they gave us one second more of sleep! MY PRECIOUSSSS", but then I realized that yes, this year there was supposed to be a leap second, but it should have been in June, not in March.

Other people I know noticed the message, but nobody knew anything else about it, and duckduckgoing didn't find anything, so I'm asking the lazyweb: does anybody know what happened?

Update: it seems that this has been traced to a single layer1 ntp server.