Wrongful convictions are where innocent people are convicted of crimes they did not commit. The causes of these convictions are complex and varied.
There are too many wrongful convictions! See this 2013 news report More than 2,000 wrongfully convicted people exonerated in 23 years, researchers say. These are cases where wrongly convicted people were exonerated, there must be many cases where this never happened.
One problem is a lack of accountability for prosecutors.
See The Untouchables: America’s Misbehaving Prosecutors, And The System That Protects Them by Radley Balko, August 2013.
Another cause is defence lawyers who fail to effectively present a case to the jury, see Bad Lawyering by Don Rehkopf, July 2014.
A summary : Causes of Wrongful Convictions at University of Michigan.
Another problem is corruption and defense lawyers with a conflict of interest America’s Corrupt Legal System www.globalresearch.ca, August 2005.
There are also specific causes, for example Eyewitnesses Are Often Wrong at http://wrongfulconvictionsblog.org/, false confessions are common and “cargo cult” science from biassed State witnesses, see David Camm , forensic fraud and NACDL: FBI overstated strength of hair evidence.