Steven Cherry ticks them off:
Twice in the last six years, the world’s timekeepers have changed our clocks by one second. And I bet you didn’t even notice, because a second is no big deal. Unless you’re an airline pilot. According to an article in the current issue of Communications of the ACM, a plane moves 300 meters in one second. A one-second hiccup in the radar of an air traffic control system is not a trivial matter. … There are no standards for how to plan and manage leap seconds, and most systems go down for planned maintenance instead of trying to deal with leap seconds in real time. The U.S. nuclear arsenal is reportedly put in a so-called special mode for one hour before and after a scheduled leap second.
The Dish 