After an extra second was added to the clock on Saturday to better align it with the Earth's rotation schedule, a whole slew of websites from LinkedIn, Foursquare and Reddit to Mozlila, Yelp and Gawker sites experienced technical issues.
Known as a "leap second," which is a basic one-second adjustment applied to the Coordinated Universal Time (UTC) to keep it up-to-date with solar time, the additional second added didn't sit well with various web servers.
http://mashable.com/2012/07/01/leap-second-bug-outages/