What such cases have in common is an act that most of us find intolerable to contemplate, such as a terrorist bombing or the murder of children. The police come under enormous pressure and, as the Court of Appeal has been forced to recognise, tend to look for two things: speedy results and the weakest link in the chain. This often means outsiders, in terms of race or class, so that six innocent Irishmen took the rap for the Birmingham bombings while a petty criminal and his associates found themselves wrongly convicted of murder in the Carl Bridgewater case.
Something similar has now happened in an American courtroom. Who could be more vulnerable, more open to suspicion in a case involving the murder of a very young child, than the 19-year-old foreigner who reported his symptoms? Louise Woodward, a young woman from a modest background - her father is a joiner, her mother works at a further education college - was already in trouble with the middle-class Boston doctors for whom she worked before their son died from injuries to his head.
What is so striking about the trial of Ms Woodward and her conviction of the murder of eight-month-old Matthew Eappen is that it displays the features of a classic miscarriage of justice: a prosecution case which relied entirely on circumstantial evidence, an unspoken but powerful class conflict, and an almost wilful disregard by the jury of compelling medical testimony produced by the defence. The purpose of the hearing at Middlesex Superior Court was to determine whether the prosecution had demonstrated beyond reasonable doubt that Ms Woodward inflicted the injuries that killed the little boy. It did not.
The child had a fractured wrist which had gone unobserved for several weeks, and it is at least possible that the fatal skull fracture was inflicted three weeks before his symptoms were sufficiently alarming for Ms Woodward to alert his parents and the emergency services. Why, then, did the jury convict her of second-degree murder? The verdict shocked not only Ms Woodward but American lawyers and observers.
A clue can be gleaned from Friday's Daily Telegraph, which explained her supposed behaviour in terms of a Jekyll and Hyde transformation: "Woodward, the adolescent-cum-little girl, moved into a prolong- ed, towering rage. Her placid, capable, if sometimes sullen-seeming exterior vanished. Instead, her hidden hatred of the Eappens suddenly focused on the crying baby."
None of this is established fact. Nor is the newspaper's account of her shaking the child for a minute or more and dashing him against a wall or floor. But the jury may have allowed itself to be swayed by some equally ghastly fantasy - for that, in the absence of evidence, is what it is - and reached a verdict with a similar picture in mind. Louise Woodward's function here is that of the stranger, welcomed into the bosom of the Eappen family, the foreigner who repays kindness by destroying the citadel into which she has been admitted; a figure graven into our imaginations by fairy-tales and myth.
She is a Trojan horse, the bad fairy at the christening, Chaucer's "smyler with the knyf" personified. She provides a focus for the anger aroused by the suffering and death of a tiny, defenceless child. She also diverts attention from the other important issue, apart from the identity of the killer, which is the inexplicably low status afforded to child care.
MATTHEW'S mother, Debbie, has said that the trial amounted to an attack on career women who pay for child care. Given the loss she and her husband, Sunil, have suffered, it is not easy to argue with her. Working mothers come in for such sustained abuse that we are acutely aware of their perpetual feelings of vulnerability, not to mention the despair Mrs Eappen must have experienced after her son's death.
Yet I am not sure that this trial is to do with motherhood. It is, surely, about the priority parents place on getting the best affordable care for their children. What has emerged is a portrait of affluent families in New England who hire inexperienced English au pairs rather than trained nannies - with the inevitable risk that injuries such as Matthew's broken wrist go undetected. Although Ms Woodward's first American employers, Lauren and Harris Komishane, speak fondly of their former au pair, they paid her around $115 (pounds 70) for a 50-hour week. When she went to the Eappens, she had to look after two young children and do light housework for pounds 1.70 an hour.
The responsibilities the Eappens placed upon her were daunting, yet they rewarded her with less than most professional couples pay their cleaners. Most of their complaints prior to Matthew's death - that she stayed out late and made long phone calls - amount to little more than the fact that they hired a 19-year-old and were surprised when she behaved like a teenager. I wonder if there is some cultural confusion here, whether American couples draw their image of English au pairs from a famous photograph of Lady Diana Spencer, also aged 19, surrounded by adoring children at the London kindergarten where she worked for a pittance, her earnings supplemented by family money.
Without a trust fund to fall back on, without proper training for a demanding job, in a foreign country where her employers treated her as a fractious child, Louise Woodward's situation before Matthew Eappen's death was far from enviable. Now it is dire, and her future depends on the tender hope that American justice will prove quicker to correct its failings than the British courts in the 1980s.