The last time a leap second was added to the calendar, a whole bunch of Websites including Gawker, Reddit, and Netflix went down, along with Qantas Airlines.

It appeared the problem was caused by a routine in Linux that was created during a long dry spell of no leap seconds, so Linux was incapable of handling the addition. The result was servers locked up.

wired.com

(The Inside Story of the Extra Second that Crashed the Web)

I presume Mr. Johnson is aware of the problem here.