Interesting example: Leo Szilard and cobalt bombs
In The Precipice, Toby Ord mentions the possibility of "a deliberate attempt to destroy humanity by maximising fallout (the hypothetical cobalt bomb)" (though he notes such a bomb may be beyond our current abilities). In a footnote, he writes that "Such a 'doomsday device' was first suggested by Leo Szilard in 1950". Wikipedia similarly says:
The concept of a cobalt bomb was originally described in a radio program by physicist Leó Szilárd on February 26, 1950. His intent was not to propose that such a weapon be built, but to show that nuclear weapon technology would soon reach the point where it could end human life on Earth, a doomsday device. Such "salted" weapons were requested by the U.S. Air Force and seriously investigated, but not deployed. [...]
The Russian Federation has allegedly developed cobalt warheads for use with their Status-6 Oceanic Multipurpose System nuclear torpedoes. However many commentators doubt that this is a real project, and see it as more likely to be a staged leak to intimidate the United States.
That's the extent of my knowledge of cobalt bombs, so I'm poorly placed to evaluate that action by Szilard. But this at least looks like it could be an unusually clear-cut case of one of Bostrom's subtypes of information hazards:
Attention hazard: The mere drawing of attention to some particularly potent or relevant ideas or data increases risk, even when these ideas or data are already “known”.
Because there are countless avenues for doing harm, an adversary faces a vast search task in finding out which avenue is most likely to achieve his goals. Drawing the adversary’s attention to a subset of especially potent avenues can greatly facilitate the search. For example, if we focus our concern and our discourse on the challenge of defending against viral attacks, this may signal to an adversary that viral weapons—as distinct from, say, conventional explosives or chemical weapons—constitute an especially promising domain in which to search for destructive applications. The better we manage to focus our defensive deliberations on our greatest vulnerabilities, the more useful our conclusions may be to a potential adversary.
It seems that Szilard wanted to highlight how bad cobalt bombs would be, that no one had recognised - or at least not acted on - the possibility of such bombs until he tried to raise awareness of them, and that since he did so there may have been multiple government attempts to develop such bombs.
I was a little surprised that Ord didn't discuss the potential information hazards angle of this example, especially as he discusses a similar example with regards to Japanese bioweapons in WWII elsewhere in the book.
I was also surprised by the fact that it was Szilard who took this action. This is because one of the main things I know Szilard for is being arguably one of the earliest (the earliest?) examples of a scientist bucking standard openness norms due to, basically, concerns of information hazards potentially severe enough to pose global catastrophic risks. E.g., a report by MIRI/Katja Grace states:
Leó Szilárd patented the nuclear chain reaction in 1934. He then asked the British War Office to hold the patent in secret, to prevent the Germans from creating nuclear weapons (Section 2.1). After the discovery of fission in 1938, Szilárd tried to convince other physicists to keep their discoveries secret, with limited success.