What are previous examples of people on LW applying mental techniques and getting into seriously harmful states?

That's not something about which I want to go publically into the details but if you would make a list of people posting about serious mental problems in the last two years and would look at the top of that list you would find the case. If you would then look at this users history you would find a discussion about applying a mental technique.

There another LW event where multiple people threw up after a few rituals. Likely no lasting damage but still not optimal.

In both cases the causality isn't obvious and you can make an argument that A wasn't the cause of B, but both of them were enough were emotional charged events and they got me to be more careful about the subject. There are also experiences that I had outside of the LW context that make me careful.

For completion there are other events where people got into mental health issues after dealing with the anthropomorphic arguments too much. Very recently a person on slack posted that they are emotionally troubled as a result of QI. But mental health issues because taking a specific road in philosophic inquiry are distinct from using arrising in connection with mental techniques.