Leap second: Linux can freeze
The leap second that was inserted on Saturday night can cause permanent high CPU loads on Linux computers. Among other examples, the behaviour is documented in the blog of the Mozilla Foundation, where strange peak loads on one Mozilla server were observed from the time the leap second was added. The developers said that the problem is easily solved by resetting the date or rebooting the system.
Leap second: Linux can freeze - The H Open: News and Features
I did file a case about this (for which I am still waiting for someone to get in touch with me - support is terrible lately).
When we noticed one of our dev servers was experiencing this, and reboot fixed it, I rebooted the production email servers later that night. And problem did not come back. Later on Sunday someone emailed me from another list about the problem, citing the Mozilla blog.