If you were using the Internet last Saturday night, you may notice some of the major sites on the Internet had problems. This wasn't due to your ISP or overwhelmed servers at your website of choice; it was likely due to the leap second invented to keep atomic time aligned with solar time. The last leap second added to clocks was on December 31, 2008.
The leap second was created by the International Earth Rotation and Reference Systems Service in 1972, and 25 have been added to world clocks since its invention. Leap seconds aren't traditionally a problem because computers can accommodate them by setting the clocks backwards by one second at the end of the day. However, technology and the Internet are much more prevalent around the world now than they were in 2008 leading to issues adding a leap second.
The problem is that any data being backed up when that leap second is added or an e-mail sent could potentially be lost. In fact, several major websites had problems Saturday night because of the leap second. Among the sites that had issues were Gawker, LinkedIn, Mozilla, Reddit, StumbleUpon, Yelp, and others. Google added milliseconds over the course of the day to keep its computers operating rather than adding an entire second at one time.