Some things you know, and some you don't. Different generations, different exposure, whatever.
On the Cruciverb-L mailing list, a constructor asked if FALSE FLAG was familiar enough to use in a Tuesday puzzle. I've never heard of the term.
A list member responded, "The term is widely known (and should be well understood by everyone, especially those whose governments engage in them.) It is certainly far less obscure than many other terms that are frequently seen in crossword puzzles." He then proceeded to give 9 paragraphs of examples.
I sent a private note to this guy: "Saying something 'should be well understood by everyone' makes anyone who hasn't heard the term feel dumb. I've never heard of 'false flag.' Luckily I have my crossword trophies to remind me I'm not dumb."
OK, maybe it was an overreaction but I don't like being told I should know something. I've since found that 2 other "Wordplay" cast members are not familiar with this term either.
And what is FALSE FLAG, you may ask? It's not a dictionary phrase. Look it up.