If what the campaign says is true, then it sounds to me like a technicality. It seems to me implausible that 22 emails containing intel of the most secret sort had that intel classified after the fact (http://www.theguardian.com/us-news/2016/jan/20/hillary-clint...).
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FROM: Important Official TO: Hillary Clinton
MESSAGE: Info you requested - JFK is still alive. He wants to meet you soon. Also, aliens. Lots of them.
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In that ridiculous example, it seems like the Sec of State isn't thinking to herself, "I hope they don't classify this info later."
It's the official's responsibility to know what is and what isn't classified whether it is "marked". As a lawyer (and a Clinton!), she would certainly be parsing her words carefully.