It wasn’t a “mistake”, it was intentional. Clinton’s only “mistake” was thinking she wouldn’t get caught.
And, it resulted in a violation of the law, by exposing classified information. The violation was compounded by not reporting the breach.
The FBI’s job is very simple. They look for classified documents dated prior to some email. They look for the same wording in an email. That is an intential spill and serious violation or felony. It is harder to look for “retroactive” classification violations which really means that Clinton violated her classificiation authority agreement by not marking her own classified statements. But they probably got her on a dozen of the former, an easy case for the prosecution.
“It wasn’t a “mistake”, it was intentional.”
Indeed, and if she thought the risk of getting caught violating these laws to hide her emails was worth it, then whatever she was hiding must have been a much bigger violation. A calculating politician like Hillary would not commit felonies trying to cover something up unless she is covering up much, much worse felonies.
The violation was compounded by not reporting the breach.
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Also compounded by her destroying evidence.