I've always wondered this, especially with the high regulation vitamins and supps have with the ever watchful eye of fellow bro science gurus.
Like how does a steroid get spilled into the pill molds or in with powders? If I was a fighter and my supplement made me piss hot, I'd be livid.
Out of all the times, I believe a small handful of these guys are actually clean sometimes.
I don't know a way to make this happen less. Fighters train a lot and not having these natural readily available support to recover during these grueling workouts like shark cartilage and such just seems unfair. Heck, I'd be afraid of tainted meats too. 
The dietary supplement industry is completely unregulated and the owner of a supplement can put in things into their supplements that act like steroids or are steroids so their users can really get results. They are usually incentivized to do so because the market for these supplements are joe shmoes and if you give them steroids they are going to get real gains and tell a friend, etc, which means you make a shitload of money.
That is why Vada or WADA has a list of over the counter supplements that contained PEDs(tainted).
So if you are an athlete you have to make sure you don't take supplements on that list and take supplements from reputable companies and have faith they aren't tainting their stuff.
https://www.youtube.com/watch?v=8dH61x9bkMc
There have been proven cases of contamination. Lawsuits resulted from it
The only problem is that supps have to be tested before getting on that list, so there'll always be victims.
They get tainted when you mix the steroids in.
when the vitamin factory is next to the Horsemeat factory, these things happen 
Op youre naive enough to think that they unintentionally add peds to supplements?
Gaspari nutrition was found putting tbol a few of their supplements years ago.
Craze pretty much had meth in it.
Hi tech pharmaceuticals was raided by the fda and they found ghb and viagra in their facility.
Its there to make the supplements work like drugs, because consumers want supplements to work like drugs.
Not a lot of companies do this obviously, but the ones that do are small fish in a big pond so its pretty hard to get caught. Every once in a while something leaks through the cracks.
Once you confirm that there are no banned substances listed in the ingredients in your supplements, tag and store a few random tablets from every bottle. If you test hot and know that you are clean.. you will at least have something to test for contamination.
To get real positive results sometimes the real deal steroids or banned substances are mixed into supplements. This seems to happen more often with new products.
If my occupation was a fighter I would make sure what I was ingesting would be legal as its my job. Blaming substances just makes u look stupid. It's your own fault for being a lazy dumb fuck.
Pretty sure it's both really some tainted by accident in factories which make loads of different shit and some intended.
ocbadapple - There have been proven cases of contamination. Lawsuits resulted from it.
Simple. I went to the factory and rubbed my taint on all of the supplements.
All of them.
AnthonyJohnsonMasterBaderedOnMyPoopyface - I've always wondered this, especially with the high regulation vitamins and supps have with the ever watchful eye of fellow bro science gurus.I'm not sure but a few years ago bodybuilding.com got in some hot water because they were selling supplements that had banned substances in them (steroids of some sort). The products that got flagged I never even heard of but I'm sure you can find information out there about it.
Like how does a steroid get spilled into the pill molds or in with powders? If I was a fighter and my supplement made me piss hot, I'd be livid.
Out of all the times, I believe a small handful of these guys are actually clean sometimes.
I don't know a way to make this happen less. Fighters train a lot and not having these natural readily available support to recover during these grueling workouts like shark cartilage and such just seems unfair. Heck, I'd be afraid of tainted meats too.
It is usually the needle in there ass that doesn't
Akston - It is usually the needle in there ass that doesn'tDoes it
hendofanforlife -Haha this
tell us more about the high regulation supps lol