We can't go to absolutes. Historically, "someone warned me off this information" has been badly counterproductive.
There are lots of warnings about information that's supposedly wrong, or confusing, but these are relatively easy information hazards to defend against. If the only danger of a text is that it's wrong, then being told why it's wrong is sufficient protection to read it. Highly confused/confusing text is a little more dangerous - reading lots of postmodernism would be bad for you - but the danger there is only in trying to make sense of it where there is no sense to be made, so, again, a warning should be sufficient defense.
I think warnings about information being actively harmful have been pretty rare, though. I can think of a few major categories, and some one-offs.
There's information that would destroy faith in a religion, and information that would alter political allegiance. These seem like obvious false alarms (since speakers have a motivation for warning falsely). In fact, the presence of a warning like that is usually evidence that you should read it.
I wouldn't call any of these classes basilisks. Information hazards, maybe, but weak ones. But then there're rare one-offs, the ones that people have called basilisks, and with confirmed deaths or psychological injuries to their credit. These are clearly not in the same league. They genuinely do require careful handling. And because they're rare one-offs, handling them carefully won't consume inordinate resources; and as long as you're making an explicit risk-benefit calculation, you can factor in the expected value of whatever it is you're blinding yourself to, so they won't blind you to very much.
Compartmentalization is bad in general, but expected utility trumps all. Every heuristic has its exceptions, and information-is-good is only a heuristic.
What I'm saying is that even though dangerous stuff is dangerous, a programme for learning to handle it strikes me as really not optional.
It seems to me that starting with analysis at a distance is a necessary (but not necessarily sufficient) precaution in that handling.
I think warnings about information being actively harmful have been pretty rare, though.
There are very few, if any, societies without censorship.
But then there're rare one-offs, the ones that people have called basilisks, and with confirmed deaths or psychological injuries to their credit. These are clearly not in the same league.
I need examples, more than the present post ("hey, here's a rambling crackpot 2000-page suicide note") or, in the case of the LessWrong forbidden post, individuals with known mental disabilities (OCD) getting extr...
My various interweb browsings stumbled me upon a potential Cockatrice in written, philisophical form. I've thus far read through the first chapter, and it is less anti-rational than most philosophical writings.
I'm reading through it right now, and will provide my feedback when I'm done, likely as a front-page post.
Personally, I'm a Fatalist, with some sort of Weird Soldier Ethic, who plans to go out the same way that Hunter did (if the cops don't get me first), but I've got a bunch of nonsense to Write first. I figure that'll make me somewhat immune. That aside, I doubt it's a real cockatrice - or we would've heard about it before.
It is a strong exercise in Nihilism. So, with those cautions given, I offer it to you: an extensive suicide letter.
Tip of the hat to this guy.