I know literally hundreds of people who regularly or semi-regularly do illicit drugs, including myself. And if you read the testing instructions, you'll find that the best way is to mix and test the entire package of drugs, not just a sample. Though to be honest, few people I know test with any regularity.
Yeah, mixing everything and testing a sample of it still does not guarantee that you won't have a hot pocket of something mysterious and deadly, but did not get into the test sample no matter how well you think you're mixing it. This is why the FDA is so hard core when it comes to approving the production of drugs. They want to know about the entire process of production from top to bottom and routinely suspend a producer's license, removing product from shelves if there is any doubt as to the safety of their end product. When parts of milligrams can mean the difference between life and death, the risk isn't worth the reward, IMHO.
