http://www.theatlantic.com/politics/archive/2010/10/failure-shuts-down-squadron-of-nuclear-missiles/65207/
October 2010:
On Saturday morning, according to people briefed on what happened, a squadron of ICBMs suddenly dropped down into what's known as "LF Down" status, meaning that the missileers in their bunkers could no longer communicate with the missiles themselves. LF Down status also means that various security protocols built into the missile delivery system, like intrusion alarms and warhead separation alarms, were offline. In LF Down status, the missiles are still technically launch-able, but they can only be controlled by an airborne command and control platform like the Boeing E-6 NAOC "Kneecap" aircraft, E-4B NAOC aircraft or perhaps the TACAMO fleet, which is primarily used to communicate with nuclear submarines. Had the country been placed on a higher state of nuclear alert, those platforms would be operating automatically because the frequencies used to transmit nuclear codes would be interfacing with separate systems, according to officials.
According to the official, engineers believe that a launch control center computer (LCC), responsible for a package of at least five missiles, usually ten of them, began to "ping" out of sequence, resulting in a surge of "noise" through the system. The LCCs interrogate each missile in sequence, so if they begin to send signals out when they're not supposed to, receivers on the missiles themselves will notice this and send out error codes.
Since LCCs ping out of sequence on occasion, missileers tried quick fixes. But as more and more missiles began to display error settings, they decided to take off-line all five LCCs that the malfunctioning center was connected to. That left 50 missiles in the dark. The missileers then restarted one of the LCCs, which began to normally interrogate the missile transceiver. Three other LCCs were successfully restarted. The suspect LCC remains off-line.
NASA:
http://www.businessweek.com/stories/2008-11-19/network-security-breaches-plague-nasa
DOE, 2012:
(not sure if this is physical security or computer security)
http://www.knoxnews.com/news/2012/sep/28/doe-moves-terminate-wsis-y-12-contract-latest-fall/?partner=popular
The Chinese are working on this 24 hours a day, every day.