It can significantly intensify your mood so if you become anxious, fearful, angry or unhappy that feeling may be so magnified as to seem overwhelming. It can often be because you took a trip while you had something on your mind, and then your thoughts about that seem monstrously distorted. Imagine being a child who gets really scared by a movie, the idea of the scary thing outweighs the context of being fictional or just a story or even the non-scary part that came before, because now your childish mind can't stop thinking about the Bad Thing.
LSD hallucinations can involve fairly severe distortions of time and space which are not helpful in that context. A trip can also last 8-12 hours which is quite a long time, so if you start feeling 'oh, I think this is too much for me..' then you're stuck with it for a long period, possibly your foreseeable future during the trip due to the extra cognitive burden of the hallucination. A dose of vitamin b12 can help (by inducing a distracting and rather pleasant hot flush), but after a few different negative experiences I realized the easiest thing to do if feeling bad was to find a quiet spot and just sit down - not too many bad things can happen to you while you're sitting still and adopting a more relaxed posture generally helps the bad mood evaporate of its own accord. Partly because of this insight, I developed an interest in Zen Buddhism and Taoism.
Excess consumption (or comsumption with other drugs like pot) can have unexpected or unhelpful effects. I've experience mild aphasia (difficulty in forming speech) one for a few minutes, and the one really bad trip I had (due to a combination or prior bad mood, excess consumption and inexperience) got unmanageable in two ways. One, I developed temporary amnesia for about an hour, and was unable to remember where I was until someone said 'you know...America?!', which was a place I could remember having heard of - that was pretty confusing, since I didn't know where I lived. Also, I had forgotten everything about my own identity except my first name, which was scary for obvious reasons. Identity is something you take for granted to such an extent that we're not really equipped to deal with not having one. Two, all this happened on a landing halfway down a long staircase, and both the up and down directions looked like the inside of a concertina that was vibrating with the loud music which was going on in the background (but which I was incapable of processing as music right then, it was just a bunch of scary noises). In short I had no idea who I was or how I had arrived in the world, and the world itself seemed chaotic and dimensionless. Eventually I got tired of feeling freaked out and the stairs settled down for a bit, so I wandered around the party in order to try figuring out my new and unfamiliar environment. Probably because I had let go of my anxiety, all my memories suddenly popped back into place - much like when you find your keys after you had lost them, sp you don't need to check every individual key on the keyring. And once that happened, I felt great for the rest of the trip. Not just better, with the satisfaction of having overcome an incredibly difficult situation.
Sounds awful, right? But what's hard to explain is that a bad (or good) trip isn't just something that happens to you; it's about how you react to your distorted perception of yourself, like the psychological equivalent of a hall of distorted mirrors. If you're prone to panic or other sorts of mental discomfort then that would obviously be pretty bad, but if you have a high tolerance of weirdness or ambiguity then it can be very rewarding, despite the existence of some dangerous or scary situations (which you could also encounter in sports or many other contexts). It's not illusory, bad or good; it's just the experience of the interaction between your brain and this particular chemical which modulates the threshold of synaptic firing. Your mind will work differently, but it's still your mind - and is the scary aspect for many people who have negative experiences, they regocnize the troubling experiences as manifestations of the subconscious.
I've tripped maybe 80 or 90 times. I stopped eventually (>10 years ago) because it felt familiar enough that I wondered if I was just exploiting it for entertainment rather than self-exploration or expansion. I would like to take some again now that time has gone by but would be more inclined to do so in quiet solitude rather than in a social context, maybe I've just come to value tranquility and calm rather than excitement as I got older. I have absolutely no regrets, even of the frightening bits - I would say that psychedelics have been among the great positive and valuable experiences in my life, not as something to be consumed and enjoyed, but as an experience to be pondered and ocntinuously re-integrated. I feel they brought a significant improvement to my mental health - I have suffered from chronic major depression since youth but was able to develop a much stronger sense of agency and larn to manage my condition thanks to my psychedelic experience.
