So, I think it's very, very rare. But I believe it can happen.
A batch of MDMA had "fallen into" my hands several years ago, a little over 10g, which is about 100 "standard" doses (though "standard" might be 150mg or 200mg these days). It was from the Netherlands (supposedly) and had been marketed as 85% purity, which sounds low but it's actually on the higher-end of the buyable spectrum. I believe much of the remaining 15% was MDA, but I don't know how I came to believe that. I applied 5 or 6 test kits (especially the specialized one that distinguishes between MDMA and MDA) and it checked out.
I never sold any, I reserved it pretty much to explore the experience with my friends, and the batch lasted probably a year and a half. (One of the friends had read somewhere that the body needed around 3 weeks to regenerate serotonin or whatever. I don't know if that's true but, who knows what's really true, and we thought 3 weeks was fair cool-down period.) All my friends at the time were software engineers, lawyers, and accountants, the left-brained type, so everyone was tracking their own personal experiments—eating before, not eating before, lower and higher dose, with and without Magnesium, Selenium, L-Tyrosine, 5-HTP, I can't remember them all. (The conclusion of that was that there was no conclusion at all, about those factors at least. I think we agreed in the end that personal expectations, mindset going into it, and who's around you, completely override food, supplements, and even minor variations of dosage.)
Anyway, where I'm getting at is I mediated between 50-70 trips (not a whole 100, since often a trip was extended with a re-up) for 15-20 unique people. Only 1 had a bad trip but the reaction was immediate. It was also repeated despite halving the dose (though maybe that was due to fear from the first instance). This friend is a friend I've known since we were in middle school. He's the funny guy in the group, quite proud of his wit. Sincerity and "soft" situations have always made him extremely uncomfortable; he shuts them down immediately with jokes and cynicism. It's just my pet theory, but, well, most can guess where I'm going with this. MDMA sort of disables your will to be insincere, it makes you naked. I think he was suddenly thrown into a world where he couldn't protect himself from _anything_ all of a sudden, helpless. And even it was just his best friend (me) and his wife, I think that actually made things way worse. He never imagined being vulnerable before, _especially_ around us, people who knew the most about him. It was a pretty bad time for him. I feel bad, but he was so quiet and just asking to be left alone that I don't think we even knew at the time how bad it was for him. He started telling us only once he was starting to feel better hours later. (But now I know how to spot a bad trip.)
I really don't think it was the MDA impurity, but I guess I can't say for sure. My belief is one _can_ have a bad trip on _mostly pure_ MDMA.
Incidentally, when I was 23, I overdosed on what I thought was MDMA but must have been mostly DXM, because I experienced levels of psychosis exactly as described for DXM. I think I'm probably kind of in denial about how much it screwed me up. I lost all those friends that year. The paranoia and auditory hallucinations never fully went away. I don't know how much of my mood disorders today were bound to happen or because of that event. Life is strange. Right now I've reached such a good place (of understanding and calm and the new people I know), and I even feel that I couldn't have reached this new me without having struggled exactly that way, that as stupid as my actions were when I was younger, I don't know if I would change anything.
