Here is the forensic side of it for those interested: Back in the 90s the lab at Fort Meade found out there was a period where they could not be sure they had good chain of custody or results on tests for a certain time period. The military dealt with it by throwing out all the tests and throwing out any court-martial results that depended SOLELY on forensic test results, which turned out to be not nearly as many cases as everyone feared.
Poppy seeds, yes, possible, same with some ethnic concoctions containing small amounts of coca materials that can be obtained from bodegas in Hispanic communities. Thing is, the nanogram levels of the people who come up with this crap are usually so stratospheric that the story is completely implausible. The legendary contact high actually is not as far-fetched as the lab expert witnesses will say, because (1) they don't have good controlled testing on it and (2) their official opinion was set in stone and stayed the same even when the detection threshold went down fivefold, i.e. what was a reasonable but unverified expert opinion at 75 ng/ml was not necessarily valid at 15 or lower ng/ml.
The samples that come up hot really are hot, though; if it tests hot chemically, the sample is test in a GC/Mass Spec to confirm the metabolite is really there before it's reported out as a positive. If it doesn't get a clear positive in the GC/MS it isn't reported out of the lab as a positive. I say metabolites because for each drug they test for metabolites (Not actually the drugs themselves) that are ONLY formed by that specific drug being metabolized in the body, though as mentioned some precursors from the same biological source as the drugs can produce very low levels of the metabolytes.
Also you can eat green or dried marijuana all day and you won't metabolize THC from it, it has to be heated to a critical temp for that (Hashish, however, is a whole 'nother story). It actually is possible to get the THC from the hash brownies, but you would know what was going on if that happened and you'd best be taking advantage of that open door policy to spill your guts or the consequences are all on you.
Most of the metabolites drop below the detection threshold within about 72 hours at 'normal' dosage levels, some even faster. THC, though, remains in the fatty tissues and for a heavy or regular user it can continue to produce a (True) positive for up to 30 days after the last toke.
I've seen separation boards occasionally give the benefit of the doubt to cases involving some very unusual facts, which are too one-off to go into here. There have also been documented cases of people going to bizarre lengths to beat the tests, including one nurse who catheterized herself to fill her bladder with clean urine before a test (She blabbed and was ratted out, then court-martialed).
I've seen a lot of courts and boards on it, but I've NEVER seen a case where I honestly believed somebody took a fall on a false positive. I'm not saying it's absolutely impossible, but then neither is hitting the PowerBall three weeks in a row.
