While each case is different and many other factors contribute, the research thus far on wrongful convictions has identified the following to be the most common reasons for why and how wrongful convictions happen.
Perjury: False statements by witnesses and informants. According to the National Registry of Exonerations, 62% of all exonerations involve perjury or false accusations.
Police and Prosecutor misconduct: prosecutors, police officers, or people involved in the investigation and prosecution of the accused abused their authority by engaging in a range of misconduct including but not limited to: withholding evidence, manipulating evidence, lying under oath (perjury), falsifying documents and misleading the jury. 57% of all exonerations involve police or prosecutor misconduct.
The most common is DNA but it also includes forensic chemistry, trace evidence examination (hairs and fibers, paints and polymers, glass, soil, etc.), latent fingerprint examination, firearms and toolmarks examination, handwriting analysis, fire and explosives examinations, forensic toxicology, and digital evidence. False forensics is evidence that is unreliable or invalid and expert testimony that is misleading. It also includes mistakes made by practitioners and in some cases misconduct by forensic analysts. “The misapplication of forensic science contributed to 52% of wrongful convictions in Innocence Project cases. False or misleading forensic evidence was a contributing factor in 24% of all wrongful convictions nationally, according to the National Registry of Exonerations, which tracks both DNA and non-DNA based exonerations.”
Nationally, 69% of DNA exonerations — 252 out of 367 cases — have involved eyewitness misidentification, making it the leading contributing cause of these wrongful convictions. Further, the National Registry of Exonerations has identified at least 450 non-DNA-based exonerations involving eyewitness misidentification. (The innocence project link below has youtube videos and graphs you can consider using)
False confessions: as discussed above, when an accused “confesses” how and why that confession came about must be scrutinized. Often times after exhaustive, coercive and fearful interrogations, people confess to get it over with it, to go home as promised or to stop the abuse.
-Saul M. Kassin