Nascar driver Bayley Curry was just handed a suspension for this very thing. Yahoo Sport's abbreviated story didn't suggest he was using it as a defense for his "positive" but he's accepting the ban.Two explantions, both of which lead to a third.
The first: they're made in industrial quantities with the manufacturing process outsourced to companies that also do other products. Sometimes, this company might be mixing a batch of, say, Methylhexaneamine . The next, they're making, say, Super Slim Fast Slam Sluprie. Becuase their cleaning processeses aren't of a high standard, some Methylhexaneamine accidentally ends up in Super Slim Fast Slam Sluprie.
The second explantion: these products only work because they contain products they shouldn't. So, say, Super Slim Fast Slam Sluprie only helps you to slim fast because of the Methylhexaneamine it contains and which the manufacturers neglected to list on their ingredients.
Both of these lead to the third exlanation: there's no regulation in the supplement industry, manufacturers can get away with murder (and some say that that's quite literally what they get away with). How you regulate, that's the question here.
None of this is actually news within the sports community, athletes know there's a problem. But still they ignore the advice being given to them. Take the Australian swimmer Shayna Jack, the current cause célèbre in supplement stories (her positive may have come from a supplement). Her IF provides information on safe supplements. Yet in December of last year Jack was promoting on Instagram a supplement not listed as being safe. Athletes, they do stupid things, no matter how many times you tell them not to.