California residents may know that in a criminal trial, the prosecution is legally obligated to disclose any information that it possesses that might be favorable to the defense. However, this is often not done, and prosecutors will sometimes claim that they did not think the evidence needed to be introduced. If such an omission is discovered, it could lead to a retrial.
This was decided in the 1963 U.S. Supreme Court case Brady v. Maryland, and the doctrine sometimes referred to as the Brady rule. One high-profile case in which it came up concerned the 2011 murder of a 6th grader. Police charged the mother's ex-boyfriend with the crime despite a lack of forensic evidence, but once the trial was underway, it emerged that the prosecution was aware of a witness who saw someone else entering the house just before the murder.
In another case, a witness described a robbery suspect as having dreadlocks. After the man was convicted, another prosecutor found a mugshot of the suspect in the file. It was taken two days before the robbery, and the man had short hair. While the man might have worn a wig, the photo was considered favorable and material evidence that had been withheld, and a new trial was ordered.
Other defendants in criminal trials may also have had their rights violated in a similar way by a prosecutor or by law enforcement. For example, evidence may have been obtained by a search conducted without probable cause. An attorney might look at constitutional issues such as these as well as the overall evidence in order to seek a dismissal of the criminal charge.