"It's a major interruption mostly because there are a lot of systems that aren't prepared to handle the leap second correctly," says Judah Levine, a physicist at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. Leap seconds occur irregularly, which makes it hard for programmers to test their fixes, he explains.
http://news.nationalgeographic.com/2015/06/150626-leap-second-earth-rotation-time-atomic-clock-science