A man who goes by the single name of Obaidullah was never convicted of of a crime, yet remained in Guantánamo for 14 years, even after charges against him were dropped in 2011.
From The Guardian:
US forces captured Obaidullah during a raid in Afghanistan in July 2002 when he was about 19. They found about 20 unactivated land mines buried in a field near his home. Authorities later concluded he was part of a bomb cell linked to al-Qaida, an allegation his lawyers have denied.
He was charged in the military tribunals in September 2008 with conspiracy and providing material support for terrorism, which appeals courts have said cannot be pursued as war crimes at Guantánamo for conduct that occurred before 2006. The government dismissed the charges in 2011 and his lawyers have been pressing for his release ever since.
Of the 80 remaining prisoners being held at Guantánamo, 28 are cleared for release.