As a graduate student working for LIGO (the Laser Interferometer Gravitational wave Observatory), one of my responsibilities was maintaining date-time code, including accounting for leap seconds, and calculating dates and time thousands of years in the past.
What is a leap second? Well, the Earth's rotation rate is slowing down. So, as time goes on the length of the day is longer. This rate is very slow. Every few years, the added length must be taken into account, much like the leap day takes into account that the orbital period of the Earth around the Sun is not exactly 365 days. When a leap second must be inserted is not predictable. The International Earth Rotation and Reference Systems Service is tasked with making observations of the Earth, and producing a twice-yearly report on whether a leap second is due to be added. This comes out by email, and on their website. The report gives a 6-month or so lead time before the leap second is inserted, at midnight on Jan 1 and on Jul 1.
So, apparently, a leap second was inserted last night: all standard atomic clocks around the world paused for a second. This caused all sorts of havoc with computers and networking devices, causing lots of sites to go down. Notably, Google was prepared.
(via Gizmodo and Wired)