If you've never heard the biochemical sleuthwork involved in the legal case of Patricia Stallings, it's definitely worth reading about: a good summary of the case can be found here.
Briefly, it involves a St. Louis-area woman who, in 1989, was sentenced to jail for the alleged poisoning of her baby son after he was hospitalized twice, and eventually died, due to apparent ethylene glycol toxicity. The diagnosis was made due to the presence of characteristic lab abnormalities (anion gap, osmolar gap), crystals deposits in the brain (felt to be oxalate crystals) as well as the documented presence of ethylene glycol--though the lab test used was apparently an older and less reliable gas chromatography method of measuring ethylene glycol in retrospect.
Fast forward several months later, when Patricia Stallings gave birth to a second child while in prison--who developed a similar illness and was diagnosed with a rare autosomal recessive error of metabolism: methylmalonic aciduria (MMA), which causes a chronic metabolic acidosis. The same type of disorder can also occur with severe vitamin B12 deficiency. Geneticists at St. Louis University and Yale University, alerted to the possibility that the first child's death might have been due to this genetic disease rather than ethylene glycol poisoning, re-examined the older blood samples, which were still available. They found that the original samples DID have an abundance of methylmalonic acid, and with a more accurate lab test did not have ethylene glycol. Patricia Stallings was determined to be innocent and subsequently released.
The legal aspects of the case, as it turns out, are just as interesting as the medical aspects: it involves the prosecuting attorney for the case actually initiating much of the detective work after the conviction which helped prove Stallings' innocence.