I've been trolling the Nexus quite a bit more recently than I have before, and notice that across the site, as well as in my own post
Jurema Wine Experience, the bad experiences are often glossed over unless someone's in need of help or the entire experience was bad. In my limited use of DMT, it seems like you're going to just have to accept the good and bad times as they come because there aren't many reports I've seen comprised entirely of just one of those two experiences.
I'll assume you're not going to read my entire experience report, but there is a part where I mention how ~60% of the experience was horrifying. I'll agree with Seeingisbelieving that DMT is just really unpredictable, and all in all, I loved the whole thing afterwards. It wasn't even my own mindset that took me to those places or pulled me out. The whole thing was a freaking roller coaster, and so I don't even know if good/bad trips apply to DMT. It seems like people mostly just forget the bad times unless they helped, and were remembered after integration; or once again, brought them down real bad. I hardly see any DMT art reflecting the bad times even though a lot of experience reports at least mention them now that I think about it.
Every other psychedelic I've done, however, is completely dependent on the individual. If you're taking a dose you know, and are ready for it (set and setting), it should be up to you to dictate whether or not the trip will be good/bad. If bad thoughts creep in, you've got to SHUT THEM DOWN! But, for me at least, after the come-up on all the other psychs, if you've kept a positive mindset, you've "made-it". Should be smooth sailing.
That being said, I've only had one trip even start to go real south on me. My buddy and I decided to take 3 tabs of acid in the morning, and after I throw mine in, he gets real quite, and says, "I'm sorry." I look at him, and ask, "About what?" But I already know. I already know. "I think they folded over," he says to me. So I play with the tabs in my mouth, and sure enough. 3 became 6, and that was not the way I wanted to start off that morning at all. I was still hungover mildly from the night before. That was a ROUGH come-up. But after an hour of pure willpower keeping me positive once it set it, I "made-it".
So aside from DMT, I'm a firm believer that you can be talked out of, or think your way out of any trip that goes bad. Others might have very different opinions.
I'll agree with Elrik that if you're going to do psychedelics, there will definitely be some rough patches. I don't see myself ever stopping though. It's the closest thing I feel to a religion.