There are a number of sources on this story. I chose Twitchy, a site that posted the story last night. During the debate last night, Hillary Clinton revealed our response time to a nuclear attack. That is classified information, and she revealed it on national television during the debate.
The article reminds us:
She also repeated on #DebateNight the established Democrat talking point that the contents of the documents posted by WikiLeaks aren’t nearly as important as the fact that Russia is hacking our servers — a threat so severe that she decided to set up her own private email server in her house.
It’s fine, though: she assured the public that just because she didn’t know what the (C) on her emails indicated, it didn’t matter as she treated everything as if it were at the highest level of classification … well, just about everything.
Please follow the link above to read the entire article. The article includes tweets from people who realized what she had said as soon as she said it. This is a degree of carelessness that would be a genuine danger were Hillary Clinton to occupy the White House.