Millions of defendants are convicted of misdemeanors in the United States each year but almost none obtain exonerations, primarily because ordinarily exoneration is far too costly and time consuming to pursue for anything less than years of imprisonment. The National Registry of Exonerations lists all known exonerations in the United States since 1989 — 2,145 cases, as of the end of 2017; only 85 are misdemeanors, 4%. In all but one of these misdemeanor exonerations the defendants were convicted of crimes that never happened; by comparison, more than three-quarters of felony exonerees were convicted of actual crimes that other people committed. In almost 80% of the misdemeanor exonerations we know about, the defendants pled guilty, compared to 16% of felony exonerations. In fact, two thirds involved defendants who pled guilty to misdemeanor drug possession in a single jurisdiction — Harris County (Houston), Texas. They were exonerated because the local forensic crime lab tests seized “drugs” even after criminal cases are closed by guilty pleas, and it found that these defendants were carrying no illegal drugs. Routine post-plea testing of alleged drugs may be unique to Harris County. Judging from these cases, however, guilty pleas by innocent misdemeanor defendants are common. Of the minority of misdemeanor exonerations that did not involve guilty pleas to drug possession, most were assaults, usually on police officers. Most of the defendants were exonerated either because videos that proved their innocence came to light after conviction, or because the police officers who testified that they had been assaulted were themselves charged with perjury, violence or other misconduct. Nearly all the few misdemeanor exonerations we know about depended on fortuitous events that made the process cheap and simple — usually forensic tests that police conducted for their own purposes; sometimes previously unknown videos, or criminal investigations of police officers. Otherwise, it seems, innocent defendants convicted of misdemeanors are just out of luck.
Gross, Samuel R. "Errors in Misdemeanor Adjudication." B.U. L. Rev. 98, no. 3 (2018): 999-1011.