The probability of a false positive in a chemical assay for a drug or poison (something appearing to show up when it isn't really there) is reasonably well known.

ECONOMIST: Whose body of evidence? | The

For decades farmers had been spraying Roundup on the ground before planting, killing all leafy weeds in its path with a chemical that conveniently broke down in the soil, so it wouldn't poison crops planted later.

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The chemical keeps the cocktail intact until it binds to a cancer cell, delivering a potent dose of anti-tumor poison.

NPR: FDA Approves New Targeted Breast Cancer Drug