People have definitely suffered lasting psychological damage, usually from having seen some lifted veil of reality that is somehow shocking, haunting or terrifying. It's not all that common though. I've read similar things about 5-MeO-DMT, people being afraid to go to sleep - just terrified about something in the experience that was traumatizing.
Not sure about the predisposition or sensitivity that can be a factor, but psychedelics can make you vulnerable and impressionable in a way that can lead to an acute development of e.g. a phobia or panic/anxiety disorder. I've gone through something pretty heavy that gave me a mosquito phobia for over a year, but then again I am predisposed to certain sensitivities (tactile sensations can bother me unusually for example). More likely seems to be to develop serious anxiety after an NBOMe experience, although it's not impossible with more traditional psychedelics. Enough people have reported issues like that here in PD with NBOMe's though, they have unreasonably strong action even besides the other health risks.
Anyway the therapeutic potential is usually much more of a standard reliable effect while the above are more rare... especially if you stop to think what it might be like if your natural sensitivities and predispositions would go into overdrive and try your best to avoid psychedelics from causing them going into overdrive. Check your set and setting and if they give you a protected situation you are unlikely to be screwed... but the most extreme psychedelics can get to you even then, if you're unlucky.
@salvia: I am fascinated by it but hardly interested anymore in taking it at all... but oral administration of super fresh leaves (no more than an hour after harvest or so) sounds interesting to me, which might involve substances in salvia that are NOT salvinorin A and degrade ultra quickly when the leaves are harvested... also quidding to just get a more relaxed dilated salvinorin A trip seems more reasonable than smoking.