Right before midnight UT on 01-Jul-2015, a leap second was inserted, making the last minute of 30th June 1 second longer than usual.. The "leap second" is adjustment to UT to compensate for the gradual slowing down of the Earth's rotation. Wikipedia can explain it far better than I can, see https://en.wikipedia.org/wiki/Leap_second.
Dean G7EOB captured the moment on video. You can download his video by clicking here here (it is a .3gp file).
Dean was using a beta version v1.01y of the forthcoming v1.01 firmware for the Shack Clock Kit. This version is capable of supporting 4-line by 20-character LCD modules such as the one seen in the video, not just the supplied 16 x 2 LCD module. It also includes other features such as a configurable local time offset, which is why the video shows the time as 1am; this is because UK Summer time is 1 hour ahead of UT.
Pausing the video at the right moment, one can obtain the picture of the exact moment of the leap second, see the photo above. This is the moment of the 61st second of the long minute right before midnight UT of 1st July 2015.
Roll on to 31-Dec-2016 and again Dean is there with his camera, ready to record the important leap second event!
Note that the reason why the clock briefly moves forward to 12:00:00 for a fraction of a second, before then displaying the leap second 11:59:60, is because the clock kit has an internal real time clock in software, so that it does not require a GPS to operate. It is this software real time clock which rolls forward to midnight, because it knows nothing of leap seconds. A fraction of a second later, the GPS serial data stream arrives, containing the time of the current second - which is then applied to the software real time clock and corrects it, showing 11:59:60 on the display.