Ness.... HPPD, PTSD... there is such a thing as... a bad trip.
Now I could really only imagine one after like 5 hits of acid or 4-7 grams in a highly charged stressful situation.. like I know about some guys that dropped acid in vietnam.. and then got attacked for real, that's a bad trip.
Some people can't handle ego disintergration. I can barely tolerate it... It's kind of like bitter medicine.
If you just take your time, and know yourself, use set and setting, they can be beautiful learning experiences.