A tooth expert came to a Staten Island court in 1998 and said the teeth in James O’Donnell’s mouth proved he had bitten a woman who had been sexually assaulted in Clove Lakes Park. Another such expert testified in upstate New York that Roy Brown’s teeth were “entirely consistent” with the bite marks found all over the body of a woman who was killed in 1991 outside a farmhouse near Auburn. Both men went to prison. Both were proved innocent later through DNA testing.

The bite-mark testimony was wrong. Mr. O’Donnell and Mr. Brown were among 21 people across the country convicted on bite-mark evidence — some of them sent to death row — who were later proved innocent by DNA tests.

It is one of the classic forensic practices, familiar to generations who watched television crime shows, that turned out to have hollow foundations when compared with the rigorous science involved in DNA testing. Besides bite marks, the techniques include comparisons of hair, handwriting, tire treads and footprints, and certain kinds of ballistic analysis.

These revelations led to efforts over the past decade by scientists and the legal authorities to take stock of investigative practices that had become embedded in the American criminal justice system since the early 20th century. Many of them resembled science but turned out to be more like magic, and untrustworthy evidence.