I'll just add my piece to this, I won't get into a big fight but I'll lay this out as I see it.
Assuming the substance someone is taking is infact LSD, with nothing added like 25X or a DOX etc. There's really very few options for what could be the difference between "dirty acid" and clean acid.
#1 Impurities like iso-LSD affecting the experience
#2 The difference in dosage is affecting the experience.
#3 People are experiencing a placebo affect when they get "dirty acid," and like any placebo, attribute it to the drug (or therapy or whatever,) and not their own mind.
#2 is a fact that I think everyone agrees with, dosage obviously matters, so 100mcgs of 100% pure LSD will of course be different than 100mcgs of 50% pure LSD. Though the purity would have to be terrible for it to affect the experience significantly (as in my example of an extremely high/unlikely difference in purity,) AND you'd have to know the mcgs per hit to make any worthwhile statement about how much real LSD you actually took, to show that 90mcgs of LSD is different then 90mcgs of LSD + 30 mcgs of iso-LSD, which isn't possible unless you made it yourself, even then you could make a small mistake.
#1 As has been stated before, the impurities have been tested alone, in quantities FAR higher than they would ever be present in a tab, with ZERO effects. Is it possible that small amounts of iso-LSD etc, alter the experience of LSD with "synergistic" effects (ignoring dosage differences?) I would say yes, it is possible, EXTREMELY unlikely, AND would go against everything that has been studied about regarding these impurities, but I would agree the possibility is > 0%
#3 Now compare #1 to #3. We all know that, even controlling set and setting perfectly, same place, time, safe food, safe drug from same batch taken at same dose, with the same mindset going in, people still can have wildly different experiences. Obviously controlling set and setting is huge, but no psychedelic is 100% predictable (other than it being unpredictable.)
That's the nature of these drugs. Now looking at the placebo affect. It's an extremely well documented phenomenon. Depending on the exact test, it will affect somewhere around 35% of people, EACH time (assuming an average placebo test, obviously 35% of cancer isn't cured by placebo.) Some people are more likely to experience the placebo affect, but NO ONE is immune from it, no matter how much you think you'd never fall for it. This has been studied over and over and over, NO ONE is immune, that is a FACT.
I'm someone who feels like I'm more resistant to a placebo than most (most people feel this way.) 3/5ths of my family are scientists (2 at PhD level, 2 big-name pharmaceuticals made by my dad + team (billions/yr for the owners decades later,)) I'm not a scientist, but work in a science setting. Have an analytical mindset, etc. Yet, when I was recovering from a minor surgery, a RN came to give me a pain medication, told me it was Toradol. I wasn't familiar, and asked what that was, and she replied that it was powerful synthetic narcotic, similar to morphine but a bit stronger and longer lasting. I've never done heroin, nor had I IV'd any opioid. But I'd done high doses of Oxy, methadone, fentanyl, dilaudid, etc, and I LOVE opioids, too much unfortunately. So, hearing that, I was pretty excited and it turned out to be one of, if not the most incredible, amazing, opioid highs I've ever felt. To this day I "flashback" to how great it was. The rush was definitely #1 among my experiences in narcotics. And I received this drug 3 (4?) times during my stay, with some minor tolerance setting in, but great recreational effects even on the final dose. Of course, after I looked it up, I discovered that Toradol is NOT a narcotic. It's basically super powerful Advil, an NSAID, (so the pain killing effects were real, the intense euphoria I experienced, pure placebo.) So, that's my little, holy shit! I got placeboed like crazy story. Point being, the placebo affect is extremely strong, and affects EVERYONE.
So if you compare #1 vs #3. We have something that has an extremely unlikely (to put it mildly) chance of being true (#1,) compared to a known phenomenon that is well studied and affects EVERYONE. So yeah, it's possible that iso-LSD is making some acid more "dirty," it's also possible that the one true God is Zeus. If you want to go on believing in dirty acid, or that the "spirit" of the chemist affects the drug (I can't even touch that, lol,) then fine whatever, you do you. Just be aware that this is a well studied area on both the impurity side, and the placebo side. And the extremely strong, consistently replicated studies, all point to #3 as being the answer (and #2 of course.) Every time.
Just really quick, I know I come off as arrogant here, and I do apologize for that, I'm positive there are things I believe in nearly 100%, that are totally false. So, apologies if I've offended anyone, I'm sure I could be proven wrong on some of my very strong beliefs. Personally, while I disagree with many beliefs, I don't blame or judge people for having them. Peace everyone