Check the full article for additional consequences of letting security certificates expire
"Various online pages run by the White House, the Federal Aviation Administration, the National Archives and the Department of Agriculture appear to be affected by the latest round of expirations, Netcraft said.The shutdown is breaking government websites, one by one | Washington Post
The report follows revelations last week that Web pages run by NASA, the Justice Department, the federal judiciary and others have been affected by a lapse in security certification. The actual number of websites affected could be much higher than 130, said Paul Mutton, a Netcraft security consultant, as some certificates may have covered multiple pages under the same agency.
The expired certificates mean that most modern Web browsers, such as Google Chrome or Mozilla Firefox, will refuse to display the pages on request — instead showing a warning message that suggests the sites may have been compromised by hackers.
In reality, nothing has happened. But security practitioners say that in another sense, that is precisely the problem."