Exile: if Communisms pre-req is "FT, nil", and your tech tree allows FT to be discovered, then the AI will be able to research Communism once it discovers FT; the human player won't. I found this out with playing Dominion Wars as a lot of the techs have FT as a prerequisite, so later on in the game the Klingons and Romulans were researching Federation designs (although I can't remember if this was me tinkering or not).
No, it doesn't crash the game, but then it's not a perfect way to isolate special technologies. The only advantage is with restricted wonders, because if a wonder of the world has a tech prerequsite with "no, no" prereqs, then the wonder won't work when built, even though it won't be obsolete. So you make the special tech prereq have a "FT, nil" prereq and the wonder works.
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Ideological conviction quickly degenerates into insistence based on ignorance.