Earlier quoted context omitted.
fwiw, to be a DEA registered lab, and thus, have known marijuana (or other controlled substances) just hanging around, you've got to keep it locked up such that it requires two people to access it, and you've got to track it (very precisely) by weight. and the DEA can check your records any time. they have caught at least some people (who were "diverting" cocaine up their nose) with this.
What is to prevent the chemist from just cutting the coke after snorting some of it. Coke comes in 10% baking soda, goes out 15% baking soda, and who is in charge of figuring that out? The person that did it...
(also chemists generally don't want some random guy's cocaine. they know the awful shit street drugs get cut with. once you're DEA registered, you can basically just order pure drugs from the government. that's the stuff that gets diverted, and it's also how you'd up the concentration of a defendant's sample.)
even so, that's why two people are supposed to be present for the handling, and samples should be homogenized early on and then split up for independent testing if the defendant wants it. add on lots of tamper evident tape, etc.