One of the most enduring myths about MDMA is the idea that the maximum possible purity for it tops out at 84%. Why 84%? Because in MDMA.HCl, only 84.1% of the content by mass is the freebase MDMA molecule, and the rest is the HCl salt. On the face of it, this doesn't make much sense. Generally when people talk about purity, they mean the absence of undesirable impurities or added cuts. A counterion isn't an impurity, and I suspect most people would prefer getting MDMA.HCl crystal or powder to having to handle oily freebase MDMA.
Indeed, this preference shows in the market. Freebase MDMA is almost never seen, and all MDMA available to consumers is MDMA.HCl. Experience reports, dosage recommendations and even scientific papers are all based on MDMA.HCl. Even harm reduction labs (with the strange exception of the Dutch) use MDMA.HCl as their reference standard, leading to many reports of MDMA with a purity above the famous 84% limit.
This myth needs to die. All it does is give advantage to dealers of low quality MDMA, who can try to claim that MDMA with purities in the low 80% range is actually of the highest quality. It can also confuse some users, who think they may need to increase their dosages to account for the "missing" 16%, not realizing all dosing recommendations already take this into account. When it comes down to it, the 84% myth is like if you are handed a glass of Coke at a party, and ask if it is just Coke to get the reply "no you don't have pure Coca Cola, you have 10% Coca Cola syrup and the rest is water". It's unhelpful and confusing when what you really want to know is if anyone has put rum in your Coke yet.