I'll go first: r/kitty. One of the hundred grillion cat subs back on Reddit, the culture in this one was you posted a cat picture, and the only word allowed in the title or in any comments or replies was "Kitty."
Someone is using that subreddit for covert communications, I just know it. Either on the level of "if u/PM_me_your_nostrils posts an orange cat, we attack at dawn!" or there's some steganography going on with the pictures, but that subreddit was too stupid to be as active as it was.
Oh, another one: anti-vaccination was pushed by health insurance companies to dampen public perception of government-run healthcare.
Vaccine development and implementation fucking worked. If people were happy with the results, they might end up swayed towards publicly-funded healthcare. So... put a lid on that by whipping up a bunch of fear, uncertainty, and doubt. Some folks will no longer see the vaccination programs as successful efforts to protect public health, but as a conspiracy to... do something. And instead of pointing to it as an example of a public healthcare program, you've first got to spend time defending evidence-based medicine, which takes up so much fucking time and energy, and ultimately won't convince people who bored too deeply into that alternate-reality tunnel.
It turned a public health initiative into a fucking tar pit, and now the once-free vaccinations cost over a hundred bucks if you don't have insurance.