I believe set and setting to be the main reason for a bad trip. Though some psychedelics do seem more predisposed to it by their nature. Mushrooms for me are the only ones I've had a truley shattering bad trip on. Ive had alot of really intense and difficult experiences with LSD. Though none I'd label as truley "bad" in the sense that the mushroom trip was.
The bad trip I had on mushrooms was mind shattering, I literally was going nuts. It was my first time and I took an eighth of some really good mushrooms and had a great time for the first hour or two. Even wondering to myself how people have bad trips. Then as time went on the movie I was watching began to bring up thoughts about life and my life and such. It went on and on getting more intense and I just couldnt handle it. I literally understood too much and it scared the fuck out of me. I learned soooo much though from the trip and began to process it afterwards. Even up to a year later I was realizing stuff I learned during it. I credit that trip for saving my life aswell. I was just doing any drug I could at that point, pill or whatever and mushrooms were just the next thing to try. They grabbed me and said, "FUCK THAT, we have plans for you." Ive researched psychedelics ever since and stayed away from anything else. Mushrooms tend to confuse people more which is why I voted for them to be more likely to cause bad trips.
Now for LSD, I have had some extremely intense experiences that I would say have been VERY difficult and have had bad MOMENTS on it. To say though I've had a bad trip on LSD though I wouldn't say. Though I do see the potential and at some point during all my trips as if to open the door and offer it to me, it shows me that true insanity is only a step away if I so choose or so stumble my way into. At this point Ive found LSD to be too powerful of a substance for me to work with. It tends to linger with me too. I get alot of HPPD from it it seems. I will still notice that im coming down more and more in the weeks after. Im not exactly a fan of this. I feel that a true bad trip on LSD would be completely destroying to the mind. One, because of the duration of the trip coupled with the extreme time dilation, that atleast I experience. And two because of the sheer intensity of a strong LSD trip. For this reason I would vote for LSD to be more likely to cause HPPD and that it has the potential for much worse bad trips.
As far as phenethylamines I have yet to have anything close to a bad trip. Ive only tried lower dose mescaline, MDMA, and 2C-b, but I have some 2C-e that I'll be experimenting with soon. Though I have gotten molly once and rolls that I think were cut with meth and it was horrible experiences.
EDIT: Kemist, your experience sounds alot like mine, except I jumped back in that weekend and had an AMAZING trip. Truely Turned me on and Tuned me in.
Step forward into your cave. That's right. You're going deeper into your cave. And you're going to find, your power animal...
Imagine your pain as a white ball of healing light. It moves over your body, healing you. Now keep this going, remember to breathe, and step forward through the backdoor of the room. Where does it lead?