Reddit, Foursquare, Mozilla, and a number of other companies were hit with technical problems Saturday evening when a single “leap second” was added to the world’s atomic clocks. And so, the “leap second bug” was born.

Atomic clocks are the keepers of precise official times. On Saturday at 5 pm Pacific time, or midnight Greenwich Mean Time, the world’s official time keepers held their clocks back by one second, in order to keep the clocks in sync with the rotation of the earth. Lots of sites went down because of that change.

The Linux operating system, the Java application platform, and many sites that depend on them were affected. They use the Network Time Protocol to keep themselves aligned with atomic clocks. When the extra second was added, the software platforms didn’t know what to do with it.

The bug hit as Amazon was recovering from the major outage of its Amazon Web Services web site outsourcing business. Companies such as Google saw the problem coming, but others did not. Reddit posted on Twitter, saying it was having problems with Java/Cassandra. Mozilla also reported a bug report, saying it had problems with Hadoop, an open-source platform based on Java. Others affected were Yelp, LinkedIn, Gawker, and StumbleUpon.

The sites are all working now, but it’s not clear what kind of downtime they had because of the leap second bug.