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3 Answers

Post-WW2, the allies sold enigma machines to developing countries, without mentioning they had broken the code.

Source: https://en.wikipedia.org/wiki/Enigma_machine

Note: some people speculate the purchasing countries knew this and accepted the risk (https://www.reddit.com/r/AskHistorians/comments/5cpqrc/after_ww2_the_uk_government_sold_enigma_machines/).

The British kept radar secret during WW2, and attributed pilots' ability to shoot down planes in the dark to vitamin A.

Source: https://www.smithsonianmag.com/arts-culture/a-wwii-propaganda-campaign-popularized-the-myth-that-carrots-help-you-see-in-the-dark-28812484/

  • Nuclear weapons
  • Dangerous biotechnology
    • Cultures of dangerous pathogens (seems a bit weird to call this a secret, but it has a lot of attributes of secrets)
    • Tools and knowledge to build better gene-editing technology
    • Concrete applications of existing technology that could be highly dangerous
  • Chemical weapons
  • Drone manufacturing technology?
  • Methods of terrorism
    • Potential critical weakpoints in infrastructure
    • Other terrorism ideas