Three prison governors in the North face disciplinary action after a violent prisoner was released by mistake, a report said.
George Damien McFerran (aged 47) serving time at Maghaberry high security jail for assault and possession of an offensive weapon, was among several inmates freed in error. He was rearrested after a weekend on the run in June last year.
He had been allowed out by a governor even though independent checks had not been done, prisons chief inspector Dr Michael Maguire said.
"Prisoner B was released primarily as a result of human error in both sentence calculation and validation checks," he said.
"The system, if correctly applied, should have been capable of preventing this mistaken release."
McFerran was re-arrested after being released from custody six weeks before he was due to be discharged.
Criminal Justice Inspection Northern Ireland's report said: "While sentence calculations can be complex in some cases, this particular case was not so."
It said the mistaken release flagged the issue of custom and practice in the checks conducted by governors, who are expected to authorise final release.
"In this case, while the release had been authorised by a governor, independent checks/calculations had not been done."
A number of staff, including three governor grades, have been served with disciplinary papers, the report said.
The report, a follow-up inspection of Northern Ireland Prison Service's mistaken prisoner releases, said another prisoner was freed on November 24 2010 because of computer error as the details which were inputted had only partly loaded. Another inmate was released erroneously on November 7 last year because incorrect data was supplied by the Courts and Tribunals Service.
The final release happened on February 8 last year and an internal Prison Service investigation has been launched but inspectors understand this is likely to be a case of human error.
All inmates have been returned to jail or their penalties otherwise discharged.
Dr Maguire said: "Encouraging work has taken place where additional focus and resources have been applied."
This includes significant effort and money spent on IT solutions.
Dr Maguire added: "However, work has been focused on longer term process re-engineering and fundamentally neglected the immediacy of the ongoing risks."
Of the 25 Northern Ireland Prison Service recommendations, 56% have been met in full, 40% have been partially completed and 4% discharged.
Dr Maguire added: "It is disappointing that formal training for front line staff has still not been delivered, with the most notable for duty governors who are expected to authorise final release.
"There is also a need for job guidance and a continued focus on the full implementation of existing controls by way of supervision and quality checks. Compliance and quality assurance with robust mechanisms must be sustained."