There are people who jumped out of the windows because they thought they could fly or that apartment was burning. There are people who ended up in lifelong psychosis due to one single (or many) trip(s). It is extremely powerful substance, much more powerful than anything usual life can bring upon you including softer drugs.
I had only the most positive and enlightening experiences with it, but can imagine many folks with deep unresolved issues (ie messed up childhood) could have a very very bad experience. Quality supervision and proper preparation should be mandatory.