The main thing that causes bad trips for me is physical pain. Usually the pain is just heartburn or indigestion, but it scares me into thinking it's my heart or lungs and that's all I start to think about.
On the other hand, being with the wrong people have also caused bad trips for me. Usually it's a trust issue however; I've been in many situations tripping where it feels like everyone's staring at me, but as long as I trust those people then it's no problem.
However, during my first experience with salvia, I tripped at home with my parents around (which turned into a somewhat bad situation in the end) and for some reason the first thought in my mind was that they had done this to me, like I failed them or did them wrong and now they had control of me. It also felt as if they were laughing at me in a very cocky way even though they weren't. It felt as if the world was on top of an open book and the pages started turning, with various parts of my memory on each page. During the experience I felt like I was at many different places ranging from the house I was born at to the place I live now. Visually, all I remember of the experience was a big colorful ferris wheel spinning while the ground was ripped from under me. The hit I took actually knocked me out and I fell down, then I got up and it felt as if the world was ripping apart from the ground up. I ran outside of my house, but it actually felt like I was forced out, and looked into the sky. I saw a bird in a tree making noises and then my image ripped in half, directly across the bird and I saw some feathers fall out. Then it felt like I was pulled back into my room. I had a song playing that made me feel like it was starting to control the experience. It seemed like the song was repeating just a few seconds during the whole experience. I then felt like I was pulled back outside, but before I made it out I had finally realized that I had smoked salvia and my parents that were standing around with with fixed smiles didn't do it to me after all. I yelled, "Dad, turn the music off!" and as soon as he turned it off, the trip stopped completely. It felt as if something was pushing my face from the side for a few minutes after it was over, though. I then went and took a shower while thinking of what happened. For some reason, I wanted to trip it again, however. Unfortunately, my dad had found the rest of the gram (I smoked a half of a gram of 20x in one big hit in my bong, without water in it) and threw it away. It was a very strange experience, but I wouldn't consider it a 'bad trip.'
A friend of mine who smoked a full gram of 20x at one time in a large bowl now complains of a speech problem that occurred directly after his experience. He doesn't really like to talk about what happened, other than saying, "It hurt really bad.", "Something was controlling me.", and "Everything was laughing at me, making fun of me for what I did." Now he has a slight stutter when he talks that I know he didn't have before the trip. He blames the salvia which I would too, because during my trip I felt as if I had a speech problem as well. I couldn't make out words and everything I tried to say to my parents came out as squeaks and jumbled words. Anyway, just thought I'd throw in my two cents about bad trips and them not necessarily being bad for me as long as complete trust is involved. Peace.