To start very broadly, I do know of instances of accidental contamination with cardiovascular drugs. I am going to be predictable and cite an equestrian example here, because of course, but some years ago, there was a rash of positives for synephrine, which is a banned substance. To quote from the FEI's final decision on one of the instances, synephrine is "a stimulant that causes vasoconstriction, increases heart rate and is used as a weight loss aid in humans" (
source). [Note that the minors involved have their names blacked out.]
It turned out that synephrine is a naturally-occurring alkaloid in plants, including teff (
Eragrostis teff), a type of grass that is widely fed to horses, as it is relatively low in sugars but relatively high in protein, and consequently is much healthier for horses than, say, oat hay or barley hay. It also turned out that teff has naturally higher levels of synephrine, and that the horses that tested positive for the cardiovascular stimulant had all received teff hay. It puts people in a bit of a pickle, because the levels of synephrine in this commonly supplied hay are not consistent, so one would have to test every delivery of teff hay for synephrine if competing nationally or internationally. The FEI can't just ban teff either, as it is an important source of equine roughage in many parts of the world, so I don't know exactly what they're going to do - probably sit on their hands and do nothing, and keep suspending people who feed teff as a source of roughage, and then going "Oops, our bad, it was teff", after a year or so of reviewing the case.
So it is possible for odd and accidental contaminations to occur.
As for trimetazidine, it is
possible for it to be detected when no intentional doping took place. I am attaching links to the abstracts of two papers on this issue.
The first, from 2015, points out that trimetazidine is a metabolite of the permitted drug lomerizine, so positive results for trimetazidine must be treated carefully:
Analytical detection of trimetazidine produced by metabolic conversion of lomerizine in doping control analysis.
The second paper, from 2018, looks at the metabolites of lomerizine, and notes that in cases of a positive test result for trimetazidine, the co-occurrence of a metabolite bis-(4-fluorophenyl)-methypiperazine is indicative of lomerizine use. On the other hand, if this specific metabolite is absent, but trimetazidine is present, then lomerizine is not the source of the trimetazidine. Abstract here:
Lomerizine, trimetazidine and bis-(4-fluorophenyl)-methylpiperazine in human urine after oral administration of lomerizine dihydrochloride. So there are ways to distinguish between the use of the permitted and the banned substances.
Whether the trimetazidine positive in this case was found with or without the metabolite, I don't know, nor am I going to hazard a guess on the likelihood of either option. I've put my foot in this already, by posting news that was unconfirmed and subsequently retracted, and this may be more of the same, even though I'm trying very hard to stick to facts!