The most noteworthy is the Zizians, a fringe group that is actually linked to several murders. Not exactly fair to lump the entirety of the site with their ideas, but I'd argue that it was a direct result of the echo chamber.

https://en.wikipedia.org/wiki/Zizians

Beyond that, Effective Altruism got its start partly on Lesswrong. Whether or not that counts as harmful would depend on how you see that movement, but one of its biggest proponents is now in jail.

https://en.wikipedia.org/wiki/Sam_Bankman-Fried

There's a famous thought experiment that came out of Lesswrong that has resulted in a lot of distress and questionable life choices from people who take it too seriously. A lot of the AI discussion I've seen on the site has the same overtones, including this particular post.

https://en.wikipedia.org/wiki/Roko%27s_basilisk